Document policies being set to NEW when including CPM.cmake (#383)

Author: iboB

README.md

@@ -131,6 +131,9 @@ Dependencies using CPM will automatically use the updated script of the outermos
 - **No pre-built binaries** For every new build directory, all dependencies are initially downloaded and built from scratch. To avoid extra downloads it is recommend to set the [`CPM_SOURCE_CACHE`](#CPM_SOURCE_CACHE) environmental variable. Using a caching compiler such as [ccache](https://github.com/TheLartians/Ccache.cmake) can drastically reduce build time.
 - **Dependent on good CMakeLists** Many libraries do not have CMakeLists that work well for subprojects. Luckily this is slowly changing, however, until then, some manual configuration may be required (see the snippets [below](#snippets) for examples). For best practices on preparing projects for CPM, see the [wiki](https://github.com/cpm-cmake/CPM.cmake/wiki/Preparing-projects-for-CPM.cmake).
 - **First version used** In diamond-shaped dependency graphs (e.g. `A` depends on `C`@1.1 and `B`, which itself depends on `C`@1.2 the first added dependency will be used (in this case `C`@1.1). In this case, B requires a newer version of `C` than `A`, so CPM will emit a warning. This can be easily resolved by adding a new version of the dependency in the outermost project, or by introducing a [package lock file](#package-lock).
+- **Some CMake policies set to `NEW`** Including CMP.cmake will lead to several CMake policies being set to `NEW`. Users which need the old behavior will need to manually modify their CMake code to ensure they're set to `OLD` at the appropriate places. The policies are:
+    - [CMP0077](https://cmake.org/cmake/help/latest/policy/CMP0077.html) and [CMP0126](https://cmake.org/cmake/help/latest/policy/CMP0126.html). They make setting package options from `CMPAddPackage` possible.
+    - [CMP0135](https://cmake.org/cmake/help/latest/policy/CMP0135.html) It allows for proper package rebuilds of packages which are archives, source cache is not used, and the package URL is changed to an older version.
 
 For projects with more complex needs and where an extra setup step doesn't matter, it may be worth to check out an external C++ package manager such as [vcpkg](https://github.com/microsoft/vcpkg), [conan](https://conan.io) or [hunter](https://github.com/ruslo/hunter).
 Dependencies added with `CPMFindPackage` should work with external package managers.
@@ -334,7 +337,7 @@ CPMAddPackage(
   NAME nlohmann_json
   VERSION 3.9.1
   GITHUB_REPOSITORY nlohmann/json
-  OPTIONS 
+  OPTIONS
     "JSON_BuildTests OFF"
 )
 ```