Don't misuse headers for bold text.

Headers are not meant to be used for emphasis -- they define the
logical structure of the document. Bold and italic are the correct
tools to use when emphasizing text.
Wrap long lines

This makes the source easier to read in, say, a terminal of standard
width. The change does not affect the rendered output since Markdown
requires paragraphs need to be separated by an empty line.
Ignore editor backup files
Fix broken link to non-existing section

The links now point to the validator and toolchain repos where the
user can actually find proper installation instructions.
Wrap more long lines
Fix broken reference to non-existing section

The "Install System Prerequisites" section has apparently been moved
to the toolchain README.
Merge pull request #257 from mgeisler/master

Cleanup README