Merge branch 'hotfix/861'

Close #861
Adding aggregate hydrator documentation
Adding note about assumed attached hydrator structure
Fixing code block ('sh' => 'bash')
Merge pull request #862 from Ocramius/feature/aggregate-hydrator

Adding aggregate hydrator documentation
[#862] rename new documentation file

- make it follow the same naming schema as other hydrator docs, so it sorts
  properly
- also, add it to BOTH the TOC and index
Merge branch 'feature/862' into develop

Forward port #862