Style Guide for Equipment Descriptions

@ShilohT proposed some guidelines for consistency in equipment descriptions (https://github.com/HabitRPG/habitrpg-shared/pull/372). This PR puts them into a README file in the same directory as gear.json.

[This is what the file will look like when viewed normally.](https://github.com/Alys/habitrpg-shared/blob/d34b1167327c41d1142518e199af6613e56e671d/locales/en/gear_README.md)

ping @SabreCat @lemoness