Improve README

Improve README

All-around improvements to the README.
Each sentence now gets its own line to eliminate ultra-long lines in the
source (expect in bullet points--separating a bullet point into multiple
lines looks weird and decreases readability).
HTTPS links are now preferred to HTTP links and used where applicable.
Link to Mercurial's website corrected to www.mercurial-scm.org
Link to Hugo's docs is now accessible with [Hugo Documentatio][] since
it is used more than once in the README.
Multiline code blocks are now fenced rather than indented and include
syntax highlighting.