Documentation: Add gcc, g++ and rsync info to installation section

[lautarodragan]

@krobi64 is this always required?

[geoffturk]

@lautarodragan @warrenv ^^^^^^^^^^^^^^^^^^^ ?

[warrenv]

rsync is not needed for explorer. We use it in node, poet-js and test-jsonld-libraries to copy json files into /dist.

[lautarodragan]

Thanks @warrenv!

[krobi64]

This is required for any application requiring the current poet-js library, or any form of the json-ld library

[krobi64]

@warrenv it will eventually be required for the explorer app unless we provide a service for the explorer to hit that returns claims already in a semantic form.

[lautarodragan]

Thanks @krobi64!

Then we do need to add it to the Installation section of the docs.

[lautarodragan]

Needs https://github.com/poetapp/poet-js/issues/250