Added "readOnly: true" to YAML reference

The readOnly configuration is documented nowhere except in the annotations reference.

I added it to the example, for a lack of a better place. But at least it will be documented somewhere.

Can you also confirm that this is correct? I'm starting to use it, and I can't find a way to have doctrine validate it is really working. Even with the metadata validation tool, it won't raise an error if I put an invalid entry (`readOnlyFOOBAR: true`) or an invalid value (`readOnly: FOOBAR`), so I'm kind of blind here.
Test EntityManager::createNamedQuery.
Test EntityManager::createNamedNativeQuery.
Update CONTRIBUTING.md
Fixing wrong key for allowing HHVM failures
Merge pull request #876 from doctrine/hotfix/hhvm-allow-failures

Fixing wrong key for allowing HHVM failures
Add failing test for DDC-1787.

Using joined table inheritance, when persisting multiple new entities
that are subclasses of a baseclass that has the @Version attribute set,
only the last persisted entity will have it's version set.
Adding tests that confirm that DDC-2844 is fixed
Adding tests to verify that dql aliases in criteria are correctly converted
Fix DDC-1787.

Credit goes to Jack van Galen for fixing this issue.
Fix for JoinedSubclassPersister, multiple inserts with versioning throws
an optimistic locking exception.
Merge pull request #875 from doctrine/DDC-2844

Adding tests that confirm that DDC-2844 is fixed
Merge pull request #842 from TheFrozenFire/tests/query-tests

Added EntityManager query creation tests
Merge pull request #832 from mnapoli/patch-4

Added "readOnly: true" to YAML reference
Merge pull request #877 from aaronmu/DDC-1787

Hotfix for DDC-1787