Add instructions about local certificate usage / certification authority trust

[ErikPelli]

There are a lot of issues (like #142 and #529) where the users complain that the proxy is not accepted by their browser because their certificate authority is not among the trusted ones.
We should add some sections in the documentation where we explain how to do that.
Related to #222.

Basically, some instructions about how to add the existing goproxy certificate to the trusted ones by the browser or to generate a new valid one and trust it (and how to use it in goproxy, citing the already existing custom CA example).

You can add all the instructions to the README of the custom CA example.

[ErikPelli]

#417

[ErikPelli]

This issue is open for anyone