Add example using checksums

[patrick-austin]

Checksums were raised as an area of particular interest at the recent demo:

These are optional, they can be part of an FTS transfer, could be created, e.g. as part of the EPAC scripts which transfer the data and added as metadata.  They can be stipulated explicitly as part of the transfer or excluded altogether.  This is for consideration by the EPAC team

The current docstrings and config descriptions should cover how this works for the API:

• If mechanism specified with request, it is used iff supported
• If mechanism and value specified with request, both are used iff supported
• Whatever is specified in the request is recorded in ICAT
• The config settings determine where the checksum is checked (source, dest, both, none) and what mechanisms are supported

All this seems to work, however a full example, similar to the existing example.md would both allow us to verify all this intended functionality and demo it to EPAC if needed. Such a document should cover:

• Only specifying a mechanism and not specifying a mechanism -> success
• Specifying a value -> success
• Specifying an incorrect value -> failure