Add Documentation for clion project generator

Change-Id: If59d4b7255fcb2718b1ff4b97246b94b4c09a8c7

docs/clion.md

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+# CLion project generator
+
+Soong can generate CLion projects. This is intended for source code editing
+only. Build should still be done via make/m/mm(a)/mmm(a).
+
+CMakeLists.txt project file generation is enabled via environment variable:
+
+```bash
+$ export SOONG_GEN_CMAKEFILES=1
+$ export SOONG_GEN_CMAKEFILES_DEBUG=1
+```
+
+You can then trigger a full build:
+
+```bash
+$ make -j64
+```
+or build only the project you are interested in:
+
+```bash
+$ make frameworks/native/service/libs/ui
+```
+
+Projects are generated in the ``out`` directory. In the case of libui, the path would
+be:
+
+```bash
+out/development/ide/clion/frameworks/native/libs/ui/libui-arm64-android/CMakeLists.txt
+```
+Note: The generator creates one folder per targetname-architecture-os combination.
+In the case of libui you endup with two projects:
+
+```bash
+$ ls out/development/ide/clion/frameworks/native/libs/ui
+libui-arm64-android libui-arm-android
+```
+
+### Edit multiple projects at once
+To combine several projects into one, you can create super projects
+and place them in:
+
+```bash
+development/ide/clion/[PATH_YOU_WANT]/..
+```
+
+These file will be symbolicaly linked in ``out/development/ide/clion``. All folders
+will also be created there.
+
+An example of a super project for surfaceflinger (using libui and libgui)
+located in development/ide/clion/frameworks/native:
+
+```
+cmake_minimum_required(VERSION 3.6)
+project(native)
+add_subdirectory(services/surfaceflinger)
+add_subdirectory(libs/ui/libui-arm64-android)
+add_subdirectory(libs/gui/libgui-arm64-android)
+```
+
+
+### Flattened filesystem
+
+Upon opening a project it looks like all the folder structure has been
+flattened:
+
+![Flattened View](before.png "")
+
+This is because you need to change the project root. Via Tools > CMake >
+Change Project Root:
+
+![Unflattened View](after.png "")
+