Improve docs for updating mainstream slugs

Lets keep this documentation closer to the codebase.

Original documentation has been updated to new usage of the higher level fabric tasks, however this rake task can document itself with a link to the main documentation for further info.
Update bsf area east-of-england to eastern
Merge pull request #327 from alphagov/update_east-of-england_to_eastern

Update bsf area east-of-england to eastern
Merge pull request #289 from alphagov/feature/improve-docs-for-updating-mainstream-slugs

Improve docs for updating mainstream slugs