Add readme

readme.md

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+This is a c++17 library to parse `*.aseprite` files. Please check out and
+support [this awesome pixelart program](https://www.aseprite.org/)!
+
+# Limitations
+
+* No support for grayscale or indexed image types. Only RGBA files are supported.
+    * Grayscale and indexed files will not parse.
+* Only "Normal" Blend Mode is supported.
+    * Blend mode is effectively ignored.
+* No support of user data properties.
+    * Files with user data properties will not prase.
+
+# Dependencies
+
+* zlib (for cel chunk decompression)
+* catch2 (for testing only)
+
+All dependencies are obtained via cmake.
+
+# How to use this library
+
+## CMake
+
+CMake integration into your own project is done via `FetchContent`.
+
+include in your `CMakeLists.txt`
+
+```cmake
+include(FetchContent)
+message(STATUS "Fetching aselib")
+FetchContent_Declare(
+        aselib
+        GIT_REPOSITORY https://github.com/Laguna1989/aseprite_lib.git
+        GIT_TAG master
+)
+
+set(ASE_LIB_ENABLE_UNIT_TESTS OFF)
+FetchContent_MakeAvailable(aselib)
+
+## .... Adding your own library and executable projects
+
+FetchContent_GetProperties(aselib)
+target_link_libraries(MyTarget aselib)
+```
+
+## Build and run tests locally (on linux)
+
+```shell
+git clone https://github.com/Laguna1989/aselib.git
+cd aselib
+mkdir build
+cd build
+cmake ..
+cmake --build .
+
+cd test/unit_test
+./aselib_test
+```