docs: Minor improvements

CODING.md
* Document the procedure for adding a new javascript class

ve.dm.MWExternalLinkAnnotation.js
ve.dm.MWInternalLinkAnnotation.js
ve.dm.ElementLinearData.js
ve.dm.LinearData.js
* Add whitespace line before preformatted code to fix a
  rendering bug

Change-Id: I54443ea3d4799328655d279f379d4ddc176c50a0