Remove extraneous newline

Remove a newline in a SRC block that branch that disrupts alignment.
Use subheadings instead of just emboldened text

Change subheadings of "Installing rtags" from just emboldened, one-line
paragraphs to Org subheadings.  e.g. "*Prerequisites*" becomes "** Prerequisites".
Add build instructions

Add a section to the README with building instructions.  We already have
setup instructions, but nothing on running the actual build.
Make formatting and capitalisation consistent

* Fill all paragraphs at column 70
* No blank line between heading and start of body
* Capitalise all occurrences of "Emacs" (except in code)
* Capitalise all occurrences of "rtags" (except in code) as "RTags" (to
  match most existing occurrences)
Add inline code markup

Put =code= markup around function names, variable names, and
string-literals in prose.
Separate sentences with two spaces

This is to improve readability when reading the raw text.
Merge pull request #258 from aidalgol/documentation

Documentation improvements