Minor updates to the readme

Hi Tom,

I had a go at running up and using this module. Seems to work quite well for my needs.

I've added some of the things I came across into the readme file so others might benefit from them.

In short:
 - permission settings for keys, puppet user need execute access it seems.
 - during my tesitng, was using an example hiera yaml file that was actually json syntax, worked with yaml backend (able to retrieve the values), but didn't work with eyaml backend. Using correct yaml syntax worked.
 - adding encrypted properties leaves behind the unencrypted version in the previous commits in the source control, something to watch out for and probably requires a password/property roll to ensure the previous version isn't usable by anyone.

I wanted to use the hiera-eyaml-gpg as well but hit the issue where additional tools were needed to build it so left it for another time.