Create changelog.md

To reduce length of readme, I'm moving release notes for versions older than the most three recent updates, here.
Create advanced-usage-techniques.md

To further decrease the length of the readme, I moved the advanced stuff here (since I also expanded on the documentation).
Update advanced-usage-techniques.md
Update changelog.md
Create gradient-test-cases.md

I migrated the test cases from the script file into its own file, to improve the test cases' discoverability and to expand upon them.
Update gradient-test-cases.md
Update to v1.2.0

Added release notes for v1.2.0, expanded/reworked documentation, and migrated less essential documentation to separate files to reduce the length of the readme.
Update to v1.2.0
Merge pull request #5 from camartinez1229/master

Update to v1.2.0, plus branch less essential info from readme into a few additional files