[doc][cmake] Convert read-me for the common CMake utils to reST

@phosek mentioned others might want it reST for consistency. As I
personally do not like Markdown at all and just did the "usual GitHub
read-me thing" out of habit, I am more than happy to oblige.

Also fix the typos found in the original.

Reviewed By: phosek, lebedev.ri

Differential Revision: https://reviews.llvm.org/D116524