Tweak to migration, README update

[dom-at-fugue]

Made a tweak and did a PR this time so we could discuss / tweak further. Here's the output at present:

scramble:credstash dom-at-luminal$ ./credstash put foo bar
foo has been stored
scramble:credstash dom-at-luminal$ ./credstash put foo baz
foo version 1 is already in the credential store. Use the -v flag to specify a new version
scramble:credstash dom-at-luminal$ ./credstash put foo baz -a
foo has been stored
scramble:credstash dom-at-luminal$ ./credstash put vers bar -v 1.1.9
vers has been stored
scramble:credstash dom-at-luminal$ ./credstash put vers baz -v 1.2.0
vers has been stored
scramble:credstash dom-at-luminal$ ./credstash put dttm bar -v 20151105
dttm has been stored
scramble:credstash dom-at-luminal$ ./credstash put dttm baz -v 20151106
dttm has been stored
scramble:credstash dom-at-luminal$ ./credstash put dttmb bar -v 2015-11-06
dttmb has been stored
scramble:credstash dom-at-luminal$ git checkout tweak-migration
Switched to branch 'tweak-migration'
Your branch is up-to-date with 'origin/tweak-migration'.
scramble:credstash dom-at-luminal$ ./credstash list
dttm  -- version 20151105
dttm  -- version 20151106
dttmb -- version 2015-11-06
foo   -- version 1
foo   -- version 2
vers  -- version 1.1.9
vers  -- version 1.2.0
scramble:credstash dom-at-luminal$ python ./credstash-migrate-autoversion.py
Skipping item: dttmb, 2015-11-06
Skipping item: vers, 1.1.9
Skipping item: vers, 1.2.0
scramble:credstash dom-at-luminal$ ./credstash list
dttm  -- version 0000000000020151105
dttm  -- version 0000000000020151106
dttmb -- version 2015-11-06
foo   -- version 0000000000000000001
foo   -- version 0000000000000000002
vers  -- version 1.1.9
vers  -- version 1.2.0

I think we have to advise people not to use if they used the -v flag for custom versioning.