Rough creation of a doc style guide

Should have started this before; important to maintain consistency.

docs/documentation_style_guide.md

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+# Yii Documentation Style Guide
+
+Guidelines to go by when writing or editing any Yii documentation.
+
+*This needs to be expanded.*
+
+## General Style
+
+* Try to use an active voice.
+* Use short, declarative sentences.
+* Demonstrate ideas using code as much as possible.
+* Never use "we". It's the Yii development team or the Yii core team. Better yet to put things in terms of the framework or the guide.
+* Use the Oxford comma (e.g., "this, that, and the other" not "this, that and the other").
+* Numeric lists should be complete sentences that end with periods (or other punctuation).
+* Bullet lists should be fragments that don't end with periods.
+
+## Formatting
+
+## References
+
+* Yii 2.0 or Yii 2 (not Yii2 or Yii2.0)
+* Each "page" of the guide is referred to as a "section".
+
+## Capitalizations
+
+* Web, not web
+* the guide or this guide, not the Guide