docs: Explain better the setup for Github

[Safadurimo]

I had problems to understand how to set the ssl parameters for github.
There is a short explanation here: https://docs.abapgit.org/guide-ssl-setup.html, but I don't have access to the SAP Notes.
I learned at #1225 that I have the option to either add these parameters to a file or use the RZ10 transaction.
I think Github access is so important, this should go into the offical documentation.

[larshp]

pull requests/suggestions welcome 😄

File to change is https://github.com/larshp/abapGit/blob/master/docs/guide-ssl-setup.md

[larshp]

In the next ABAP trial version the parameters should already be there, https://twitter.com/se38/status/1029281064446058496

[larshp]

@Safadurimo will you add a PR with suggestions?