document the use of shebang line in mmd files

[ssbarnea]

By adding a shebang line to the first line of the diagrams we should be able to build them by executing them.

#!/usr/bin/env mermaid

I tested and it seems to work but it would be really useful if this usage would be documented and promoted in usage examples.

Not related to this bug but I was not able to find a way to specify the filename of the output file, which seems to contain the mmd extension too (my-diagram.mmd.png) -- which is not really ideal.