_s: Add context for translations and styles.

The referenced items are decisions we made for `_s`, that a re likely to be brought up again in pull requests or issues. Since our stance on them is not likely to change any time soon, let's include them in the Contributing FAQs.

See #464.

CONTRIBUTING.md

@@ -8,6 +8,7 @@ Here are some tips to consider and to help you write a great report:
 * `_s` supports Internet Explorer 9 and greater, as well as the latest two versions of all other major browsers.
 * `_s` is backwards compatible with the two versions prior to the current stable version of WordPress.
 * `_s` uses HTML5 markup. In HTML5, it is common to use multiple `<h1>` elements.
+* We decided to not include translations [[#50](https://github.com/Automattic/_s/pull/50)] beyond the exisiting `_s.pot` file, a RTL stylesheet [[#263](https://github.com/Automattic/_s/pull/263)], or editor styles [[#225](https://github.com/Automattic/_s/pull/225)], as they are likely to change during development of an `_s`-based theme.
 
 By contributing code to `_s`, you grant its use under the [GNU General Public License v2 (or later)](http://www.gnu.org/licenses/gpl-2.0.html).