Clarify how to use the keystore from outside the project

[skumargithub]

I noticed that people were struggling with how to get the keystore to work.

It can be in the resources folder of the project or in any location on the filesystem.
The way to access them is just a bit different.

This commit makes it super clear.

[sirthias]

Nice, thank you for this patch!

Before being able to merge it we'd need to accept the Typesafe CLA, which can easily be done online.

[skumargithub]

OK, done!