Change "sanctioned" to "approved" (#3011)

* Change "sanctioned" to "approved"

Clarify the meaning of the list in the Readme.md.

* Change "Approved" to "Supported"

Suggestion from #3011.

There are other options in the comments still. I'm not really sure how to come to consensus in an open source project on a qualitative change like this.

Author: t-mart

Readme.md

@@ -20,7 +20,7 @@ ship with a command line interface (CLI) you can use directly. There is also
 [SassC](https://github.com/sass/sassc), which is the official lightweight
 CLI tool built by the same people as LibSass.
 
-### Excerpt of "sanctioned" implementations:
+### Excerpt of Supported Implementations:
 
 - https://github.com/sass/node-sass (Node.js)
 - https://github.com/sass/perl-libsass (Perl)