Document the version of clang-format used

Different version of clang-format may format files differently, which
would cause CI issues in the linting job if there is a mismatch.

This commit documents in the README that version 11 is the one that is
used.

Fixes: #32

Author: friendlyanon

README.md

@@ -73,7 +73,7 @@ Make sure you have these programs installed:
 * [cppcheck](#cppcheck) (optional)
 * [Doxygen](#doxygen) (optional)
 * [LCOV](#lcov) (optional)
-* [clang-format](#clang-format) (optional)
+* [clang-format 11](#clang-format) (optional)
 * [codespell](#codespell) (optional)
 
 ---
@@ -166,6 +166,9 @@ used to enforce style guides.
 Two targets are made available to check and fix code in developer mode using
 the `format-check` and `format-fix` targets respectively.
 
+**NOTE**: the project generates files that are formatted according to
+clang-format 11. Newer or older versions may format the project differently.
+
 ### codespell
 
 [codespell][15] is a tool to find and fix spelling errors mainly in source