README.md: Add warning about changes in 3.0 and branch info

README.md

@@ -3,12 +3,29 @@
 | [![Build status][Master image]][Master] | [![Build status][2.7 image]][2.7] | [![Build status][2.6 image]][2.6] | [![Build status][2.5 image]][2.5] |
 | [![Coverage Status][Master coverage image]][Master coverage] | [![Coverage Status][2.7 coverage image]][2.7 coverage] | [![Coverage Status][2.6 coverage image]][2.6 coverage] | [![Coverage Status][2.5 coverage image]][2.5 coverage] |
 
+ ##### :warning: You are browsing the code of upcoming Doctrine 3.0.
+ ##### Things changed a lot here and major code changes should be expected. If you are rather looking for a stable version, refer to the [2.6 branch][2.6] for the current stable release or [2.7 branch][2.7] for the upcoming release. If you are submitting a pull request, please see the _[Which branch should I choose?](#which-branch-should-i-choose)_ section below.
+
+-----
+
 Doctrine 3 is an object-relational mapper (ORM) for PHP 7.2+ that provides transparent persistence
 for PHP objects. It sits on top of a powerful database abstraction layer (DBAL). One of its key features
 is the option to write database queries in a proprietary object oriented SQL dialect called Doctrine Query Language (DQL),
 inspired by Hibernate's HQL. This provides developers with a powerful alternative to SQL that maintains flexibility
 without requiring unnecessary code duplication.
 
+-----
+
+### Which branch should I choose?
+
+ * I am submitting a bugfix for a stable release
+   * Your PR should target the [lowest active stable branch (2.6)](2.6).
+ * I am submitting new feature
+   * Your PR should target the [master branch (3.0)][Master].
+ * I am submitting a BC-breaking change
+   * Your PR must target the [master branch (3.0)][Master].
+   * Please also try to provide a deprecation path in a PR targeting the [2.7 branch][2.7].
+
 
 ## More resources: