Document and implement cmake dependency bump to 3.5.1

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 2.8.12)
+cmake_minimum_required (VERSION 3.5.1)
 
 project (benchmark)
 

README.md

@@ -54,6 +54,9 @@ sudo apt-get install git
 sudo apt-get install cmake
 ```
 
+_See [dependencies.md](dependencies.md) for more details about the supported
+versions of build tools._
+
 Now, let's clone the repository and build it
 
 ```

dependencies.md

@@ -0,0 +1,16 @@
+# Build tool dependency policy
+
+To ensure the broadest compatibility when building the benchmark library, but
+still allow forward progress, we require any build tooling to be available for:
+
+* The last two Ubuntu LTS releases AND
+* Debian stable
+
+Currently, this means using build tool versions that are available for Ubuntu
+16.04 (Xenial), Ubuntu 18.04 (Bionic), and Debian stretch.
+
+## cmake
+The current supported version is cmake 3.5.1 as of 2018-06-06.
+
+_Note, this version is also available for Ubuntu 14.04, the previous Ubuntu LTS
+release, as `cmake3`._