(this depends on an unconfirmed API in librdkafka, subject to change)

* Bumps librdkafka for the Docker build to 0.11.1 final

* Adds the -F <key> flag, through happyhacking by a C noob

* Caches key_len for fixed key

* Changes the opt from -F to -k

* Documents the relation between -K and -k

* Clarifies that you can get the fixed key for some messages

Add Dockerfile

This reverts commit cafd5e2a.

we build master anyway so the code moves too

Fix typo in readme