Compiler Explorer Instructions: a few tweaks

[dietmarkuehl]

When trying to follow the Compiler Explorer Instructions I encountered a few minor issues:

1. Cloning Compiler Explorer repos says to clone the repositories. The actual Compiler Explorer repo will need to be changed (if I recall correctly) and it makes sense to start off forking this repo.
2. In Update compiler-explorer it states to find etc/config/c++.local.properties. I think using <compiler-explorer-dir>/etc/config/c++.local.properties would have helped me locating it. Also, I think an example got added since but that is helpful.
3. I think a screenshot circling the location of the "add the library" think would be really helpful.

[neatudarius]

Hello! Thanks for feedback. @dietmarkuehl , are you willing to apply the changes yourself?