Reading for content; editing for consistency.

Corrected the incorrectly used word 'thought' to 'through'. 
Corrected grammar of 'how to write an extensions'. 
Clarified the meaning of 'ensuring that you really need to start coding'.
Added full stops to 'aka' abbreviation to make it clearer it is an abbreviation.
Changed 'developer environment' to more conventional and accurate phrase 'development environment'.

Removing "Other sources of information" section from home page because that content is now in "Developer Community" page

perl -0777 -i -pe 's|^(.*)\n-{3,}$|## $1|gm' `find . -name '*.md'`
perl -0777 -i -pe 's|^(.*)\n={3,}$|# $1|gm' `find . -name '*.md'`

Large scale clean up of markdown code (in almost all files) to conform to markdown conventions that I observed to be used consistently in the official CiviCRM User Guide. These convensions are now documented in the markdownrules.md file

Added link to Wiki so we can link to these docs rather than the wiki for the civicrm.org downloads page.

A followup on PR 1.  I changed the test to testing.