Tweak layout in README.md installation markup.

Some code blocks weren't indented, so they ended up messing up the numbering. Indenting them fixed it.
Merge pull request #832 from craigcitro/patch-1

tweak layout in readme installation markup
Fix small misspelling: Mardown -> Markdown
Merge pull request #883 from seanfisk/fix-small-misspelling

fix small misspelling: Mardown -> Markdown
add contribution guidelines from dev
Merge branch 'master' of github.com:hakimel/reveal.js