add "add badge to your project" section

[jbmoelker]

Summary

Proposed changes:

• add "add badge to your project" section. incl. summary, why and how with copy-paste code snippets.

  Checklist

• [X] The changes only affect or contain one style guide rule or section.
• [X] The changes are in style guide format.
• [X] The new style guide section has an entry in the Table of Contents (only if changes introduce new section).
• [X] The changes can be merged into the target branch without conflicts.

[jbmoelker]

@phortuin relative protocols have become an anti-pattern:

Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique is now an anti-pattern. If the asset you need is available on SSL, then always use the https:// asset. -- Paul Irish

[jbmoelker]

@phortuin in response to https://github.com/voorhoede/riotjs-style-guide/pull/41#discussion_r57727716:

Considered as separate points. Felt like a bit much. Prefer bullet list? I'm happy to change it.

[phortuin]

@jbmoelker in response to #41, leave it, making it bullets would suggest that they are actual "pros" instead of just our kind request to use our stuff