DOC: Provide documentation for cmake best practices.

build/cmake/README.md

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+# Cmake contributions
+
+Contributions to the cmake build configurations are welcome. Please
+use case sensitivity that matches modern (ie. cmake version 2.6 and above)
+conventions of using lower-case for commands, and upper-case for
+varibles.
+
+# CMake Style Recommendations
+
+## Indent all code correctly, i.e. the body of
+
+ * if/else/endif
+ * foreach/endforeach
+ * while/endwhile
+ * macro/endmacro
+ * function/endfunction
+
+Use spaces for indenting, 2, 3 or 4 spaces preferably. Use the same amount of
+spaces for indenting as is used in the rest of the file. Do not use tabs.
+
+## Upper/lower casing
+
+Most important: use consistent upper- or lowercasing within one file !
+
+In general, the all-lowercase style is preferred.
+
+So, this is recommended:
+
+```
+add_executable(foo foo.c)
+```
+
+These forms are discouraged
+
+```
+ADD_EXECUTABLE(bar bar.c)
+Add_Executable(hello hello.c)
+aDd_ExEcUtAbLe(blub blub.c)
+```
+
+## End commands
+To make the code easier to read, use empty commands for endforeach(), endif(),
+endfunction(), endmacro() and endwhile(). Also, use empty else() commands.
+
+For example, do this:
+
+```
+if(FOOVAR)
+   some_command(...)
+else()
+   another_command(...)
+endif()
+```
+
+and not this:
+
+```
+if(BARVAR)
+   some_other_command(...)
+endif(BARVAR)
+```
+
+## Other resources for best practices
+
+`https://cmake.org/cmake/help/latest/manual/cmake-developer.7.html#modules`