Add CASR as third party tool in docs.
MODIFY
docs/third_party_tools.md
docs/third_party_tools.md
generates builds of debian packages suitable for AFL.
* [afl-fid](https://github.com/FoRTE-Research/afl-fid) - a set of tools for
working with input data.
+* [CASR](https://github.com/ispras/casr) - a set of tools for crash triage and
+ analysis.

Add CASR as third party tool in docs.