Update css.md

Spelling
Merge pull request #42 from mattumotu/patch-1

Update css.md
Spelling: manor => manner

Also removed trailing whitespace.
Merge pull request #43 from alphagov/spelling_of_manner

Spelling: manor => manner
Document 202 status code behaviour

This is implemented in Rummager already, and is a common pattern, so
make it canon.
Copy linking style from 202

I feel this is a better pattern for linking as the link text is more
direct and related to the surrounding text.
Merge pull request #44 from alphagov/add-202-status-code

Add 202 status code
Add initial guide to using pull requests

This guide is intended to help us write better pull requests, and to be
better reviewers when helping to merge things.

It's basically complete as-is but could do with some examples.
Pull requests: cross-linking related PRs
Make pairing/2nd pair of eyes distinction explicit

It was pointed out that this item was unclear under pairing
circumstances - we need an 'independent' 3rd party to do the merge, so
pairs shouldn't merge each other's work.
Add line about running code
Add extra section on praising commits
Merge pull request #45 from alphagov/pull-requests

Add initial guide to using pull requests
Fix typos in "pull requests" styleguide.
Merge pull request #46 from alphagov/fix-typos-on-pr-styleguide

Fix typos in "pull requests" styleguide.
RFC2616 is dead, y’all

https://www.mnot.net/blog/2014/06/07/rfc2616_is_dead
Merge pull request #47 from alphagov/feature/update-rfc-link

RFC2616 is dead, y’all
Fix minor markdown issue in Ruby style guide
Merge pull request #48 from chrishol/patch-1

Fix minor markdown issue in Ruby style guide
Add PR guideline for ensuring READMEs are up to date