WIP Vendoring documentation

docs/v2-system-addon/vendor.md

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+# Vendoring in npm libraries
+
+#### Build system basics
+
+#### Vendored libraries as externals
+
+#### Is this library a good candidate to depend on and vendor?
+
+#### How to vendor in a library
+
+- [ ] file a bug for vendoring in the library itself
+
+- [ ] Get license approval from the licensing team
+
+  - [ ] review the mozilla-central policy at
+        https://www.mozilla.org/en-US/MPL/license-policy/
+  - [ ] file a license bug as described in the policy doc to request approval
+  - [ ] make it block the next upcoming export bug (if that bug is not already
+        filed, file one)
+  - [ ] post a patch to add the license to
+        [ `toolkit/content/license.html` ](https://searchfox.org/mozilla-central/source/toolkit/content/license.html)
+  - [ ] get the patch reviewed
+  - [ ] make sure that the patch gets landed at export time.
+
+- [ ] Install the package using npm
+
+  - [ ] Check that you're using npm 6.9.0 with `npm --version`
+  - [ ] Be sure to save the exact version by passing `--save-exact` to
+        `npm install`
+
+- [ ] Include the vendored files and reference them
+
+  - [ ] update [`bin/vendor.js`](../../bin/vendor.js) to copy any files
+        needed at runtime as well as the license to the [`vendor/`
+        directory](../../vendor/)
+  - [ ] execute `bin/vendor.js` to copy your files into place so they get
+        committed and referenced at runtime
+  - [ ] Add the exported symbol for your library to the list of externals in
+        [`webpack.system-addon.config.js`](../../webpack.system-addon.config.js)
+        so that webpack knows where to link the symbol from
+  - [ ] Add the library to the list of scripts in
+        [`bin/render-activity-stream-html.js`](../../bin/render-activity-stream-html.js)
+        so it gets loaded at runtime
+  - [ ] test and make sure that everything builds and works at runtime