[ANNOUNCE] Release branch 1.7 has been created

[Till Rohrmann]

Dear community,

the release-1.7 branch has been created [1]. This unblocks the master from
merging new features into it.

If you are working on a 1.7 relevant bug fix, then it is important to merge
it into the release-1.7 and master branch.

[1] https://github.com/apache/flink/tree/release-1.7

Cheers,
Till