Add David Black's The Well-Grounded Rubyist
[#14] Apply template to TDD trail
[#21] Identify paid resources with dollar sign
[#13] Apply template to Ruby trail
* Add Ruby Koans
* Add sections of the Pickaxe
[#17] Add Michael Hartl's Rails Tutorial
[#12] Apply template to refactoring trail
[#20] Add gitready as critical learning for git
* Add advanced git validation.
[#15] Add gitcasts for git critical learning
* Good for visual learners.
* Restrict to normal workflow and distributed workflow.
[#10] Apply sections template to Vim trail
[#7] Apply sections template to Rails trail
* Use relative links in README.
[#6] Apply sections template to Postgres trail
[#3] Create template for trail sections
* Use git trail as template.
* Name sections "Critical learning", "Validation", "Ongoing Reference".
[#2] Improve shell command formatting
[#1] Import from internal wiki