(docs) Add documentation regarding setting up appveyor builds

CI.md

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+# Setting up Cake Contrib Addins on CI providers
+
+<!-- markdownlint-disable -->
+<!-- TOC depthfrom:2 depthto:6 -->
+
+- [What is this](#what-is-this)
+- [Using AppVeyor](#using-appveyor)
+  - [Available Environment Variables](#available-environment-variables)
+  - [Using Cake.Recipe 2.x with AppVeyor](#using-cakerecipe-2x-with-appveyor)
+  - [Using Cake.Recipe 1.x with AppVeyor](#using-cakerecipe-1x-with-appveyor)
+    - [Additional notes](#additional-notes)
+- [Using Azure Pipelines](#using-azure-pipelines)
+
+<!-- /TOC -->
+<!-- markdownlint-enable -->
+
+## What is this
+
+The goal of this document is to help setting up new builds for different build
+providers under the cake-contrib organization.
+Some CI providers may already have Environment Variables or secrets set on the
+organization level that are available during the build.
+
+## Using AppVeyor
+
+### Available Environment Variables
+
+The following environment variables are accessible when running builds on the
+AppVeyor CI provider and will be available without further configuration.
+Private variables are noted, which means they will no be available in pull requests.
+However, these private variables can be made available in pull requests
+originating from the same repository.
+
+- `APPVEYOR_API_TOKEN` (_secret variable_)
+- `AZURE_PASSWORD`
+- `AZURE_SOURCE`
+- `AZURE_USER`
+- `GITHUB_PASSWORD` (_secret variable, can be used as a GitHub Token_)
+- `GITHUB_USERNAME`
+- `GITTER_ROOM_ID` (_secret variable_)
+- `GITTER_TOKEN` (_secret variable_)
+- `GPR_PASSWORD`
+- `GPR_SOURCE`
+- `GPR_USER`
+- `MYGET_API_KEY` (_secret variable_)
+- `MYGET_SOURCE`
+- `NUGET_API_KEY` (_secret variable_)
+- `NUGET_SOURCE`
+- `TWITTER_ACCESS_TOKEN_SECRET` (_secret variable_)
+- `TWITTER_ACCESS_TOKEN` (_secret variable_)
+- `TWITTER_CONSUMER_KEY` (_secret variable_)
+- `TWITTER_CONSUMER_SECRET` (_secret variable_)
+- `WYAM_ACCESS_TOKEN` (_secret variable_)
+- `WYAM_DEPLOY_BRANCH`
+
+### Using Cake.Recipe 2.x with AppVeyor
+
+While the latest version of Cake.Recipe is currently in pre-release stage,
+it is entirely usable on AppVeyor.
+We already assume you have configured the repository to use Cake.Recipe,
+otherwise please see its [Getting Started][cake-recipe-started] on how to proceed.
+
+_Please see the [Cake.Recipe documentatian][cake-recipe-migrate] on how to upgrade
+from Cake.Recipe 1.x te 2.x._
+
+If you do not need to specify any additional configuration, and wishes to just
+have it work with default options.
+Then the most basic build script can be specified as follows:
+
+```yml
+# This image may be changed when needed,
+# but the recommendation is to keep it on
+# Visual Studio 2019.
+image: Visual Studio 2019
+
+#---------------------------------#
+#  Build Script                   #
+#---------------------------------#
+build_script:
+  - ps: .\build.ps1 -Target CI
+
+# Tests
+test: off
+
+#---------------------------------#
+#        Branches to build        #
+#---------------------------------#
+branches:
+  # Whitelist
+  only:
+    - develop
+    - master
+    - /release/.*/
+    - /hotfix/.*/
+
+#---------------------------------#
+#  Build Cache                    #
+#---------------------------------#
+cache:
+  # Update this cache variable to match
+  # your current setup.
+  - "tools -> recipe.cake,tools/packages.config"
+
+#---------------------------------#
+#  Environment Variables          #
+#---------------------------------#
+
+environment:
+  # We set MYGET_SOURCE to an empty variable due to known issues when pushing
+  # packages to MyGet
+  MYGET_SOURCE:
+```
+
+**WARNING: At this time it is not recommended to add a matrix to build on
+multiple platforms on AppVeyor due to the increased build time necessary.**
+
+### Using Cake.Recipe 1.x with AppVeyor
+
+Configuring AppVeyor to build projects targeting Cake.Recipe 1.x is very similar
+to how to use Cake.Recipe 2.x documentation.
+The main change between the build script on AppVeyor is mainly to change the
+build target from `CI` to `AppVeyor`.
+See the below build script as an example of this.
+
+```yml
+image: Visual Studio 2017
+#---------------------------------#
+#  Build Script                   #
+#---------------------------------#
+build_script:
+  - ps: .\build.ps1 -Target AppVeyor
+
+# Tests
+test: off
+
+#---------------------------------#
+#        Branches to build        #
+#---------------------------------#
+branches:
+  # Whitelist
+  only:
+    - develop
+    - master
+    - /release/.*/
+    - /hotfix/.*/
+
+#---------------------------------#
+#  Build Cache                    #
+#---------------------------------#
+cache:
+  # Update this cache variable to match
+  # your current setup.
+  - "tools -> recipe.cake,tools/packages.config"
+
+#---------------------------------#
+#  Environment Variables          #
+#---------------------------------#
+
+environment:
+  # We set MYGET_SOURCE to an empty variable due to known issues when pushing
+  # packages to MyGet
+  MYGET_SOURCE:
+```
+
+**WARNING: At this time it is not recommended to add a matrix to build on
+multiple platforms on AppVeyor due to the increased build time necessary.**
+
+#### Additional notes
+
+If you do not plan to use AppVeyor as the preferred build platform,
+it may not be possible to use Cake.Recipe 1.x. Please migrate to the latest 2.x
+pre-release instead.
+
+## Using Azure Pipelines
+
+There is currently no documentation on how to set up Cake Contrib addins for
+building on Azure Pipelines.
+
+<!-- The following migrate docs need to be
+  updated when migration documentation are
+  complete -->
+
+[cake-recipe-migrate]: https://github.com/cake-contrib/Cake.Recipe/issues/612
+[cake-recipe-started]: https://cake-contrib.github.io/Cake.Recipe/docs/usage/getting-started-with-cake-recipe