Add note about required file naming convention

[David Hill]

I had some head-scratching before I realised that my file had to start with mock_

[Ludovic Fernandez]

Hello,

This constraint is explained in the "How to use" section: https://github.com/traefik/mocktail#how-to-use

[David Hill]

It doesn't specify that this is a required naming convention.
if it wasn't obvious to me then it might not be obvious to the next person

[Ludovic Fernandez]

If you have a suggestion to improve this section, I will be happy if you create a PR on this.

[David Hill]

Oh, the the file must be named mock_test.go and nothing else

ok, yea, I will create a PR