docs: add some text on HA

Signed-off-by: David Scott <dave.scott@citrix.com>
docs: add some blurb about DR

Signed-off-by: David Scott <dave.scott@citrix.com>
docs: add some text on RPU

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #13 from djs55/lots-of-docs

Docs on HA, DR and RPU