README: Replace ‘php-mode’ with ‘PHP Mode’ for consistency

For some months now I have been writing ‘PHP Mode’ for the name of the
project and so I wanted to update the README accordingly.
README: Update the ‘Status’ section
README: Use the spelling ‘GitHub’ consistently

I realize how marginal of a detail this is, but that is the correct
way to spell it.  Patches like these are a clear sign that I’m
avoiding legitimately important issues.
README: Add installation instructions
Address the compiler warning regarding `c-syntactic-context'