add readme

README.md

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+[![Actions Status](https://github.com/TheLartians/Ccache.cmake/workflows/CI/badge.svg)](https://github.com/TheLartians/Ccache.cmake/actions)
+
+# Ccache.cmake
+
+Ccache for CMake
+
+## About
+
+Ccache.cmake makes it easy to use [_Ccache_](https://ccache.dev) inside a _CMake_ project.
+It adds an addition configuration option `USE_CCACHE` that, when enabled, configures CMake to compile sources using the ccache compiler cache.
+It is compatible with _Makefile_, _Ninja_ and _Xcode_ targets.
+
+After [integrating](#how-to-integrate) _Ccache.cmake_, you can configure your project to compile with _Ccache_ with the following command.
+
+```bash
+cmake . -DUSE_CCACHE=YES
+```
+
+## How to integrate
+
+### Using [CPM.cmake](https://github.com/TheLartians/CPM) (recomended)
+
+Run the following from the project's root directory to add CPM to your project.
+
+```bash
+mkdir -p cmake
+wget -O cmake/CPM.cmake https://raw.githubusercontent.com/TheLartians/CPM/master/cmake/CPM.cmake
+```
+
+Add the following lines to the project's `CMakeLists.txt` after calling `project(...)`.
+
+```CMake
+include(cmake/CPM.cmake)
+
+CPMAddPackage(
+  NAME Ccache.cmake
+  GITHUB_REPOSITORY TheLartians/Ccache.cmake
+  VERSION 1.0
+)
+```
+
+### Using git submodules (not suited for libraries)
+
+Run the following from the project's root directory.
+
+```bash
+git submodule add https://github.com/TheLartians/Ccache.cmake 
+```
+
+In add the following lines to the project's `CMakeLists.txt` after calling `project(...)`.
+
+```CMake
+add_subdirectory(Ccache.cmake)
+```
+
+## Dependencies
+
+_Ccache.cmake_ requires _CMake_ and _cache_.