#62 doctrine/phpcr-odm#582 - adding transaction chapter
#62 doctrine/phpcr-odm#582 - describing `Doctrine\ODM\PHPCR\DocumentManager#transactional()` API
#62 doctrine/phpcr-odm#582 - notes on current limits of the `Doctrine\ODM\PHPCR\DocumentManager#transactional()` API
#62 doctrine/phpcr-odm#582 - referencing transactions chapter in the index page
Merge pull request #65 from Ocramius/feature/doctrine/phpcr-odm#582-#64-transaction-docs

`DocumentManager#transactional()` docs