Defined stable version point of Doctrine.
merged branch guilhermeblanco/patch-14 (PR #7161)

This PR was merged into the 2.1 branch.

Commits
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06ebb0d Defined stable version point of Doctrine.

Discussion
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Defined stable version point of Doctrine.

As per @stof suggestion, updated 2.1 branch to define the stable version point for Doctrine libraries. @fabpot it's all yours! =)

```
| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
```