add spark credential decryption steps to release notes

Now that the template for the release notes are ready ensure the following documentation is there.

I've pushed some code to master for spark credential decryption that requires a NiFi config change for it to work. Please edit /opt/nifi/current/bin/nifi-env.sh in your sandbox and add this line to the end:

export ENCRYPT_KEY=<paste value from /opt/kylo/encrypt.key>