fix bad contributors link by changing trail_map to trail-map
* Start HTML trail. * Start CSS trail. * Add content to typography, grids, visual design, and information design. * Fix Amazon links. * Add validation to information design. * Add HTML and CSS trails to README. * Remove "Know" in Critical Learning sections. * Add "Use vocabulary when..." validations.
* Start grids trail * Start typography trail. * Start visual design trail. * Start information design trail.
* Link to Dr. Nic's classic 8 steps article. * Link to SemVer. * Separate using open source, contributing open source, and maintaining an open source project. * Promote NEWS and ChangeLog files. * Promote SemVer.
* Added CONTRIBUTING.md file with guidelines * Created link in README.md to contributing page