[DOC] Add info: how to sync forks

[ericmjl]

Brief Description of Fix

Currently, the docs do not provide instructions on how to sync up pyjanitor forks. This should be added as it is difficult for newcomers to know how to keep things updated.

Relevant Context

• This page should be edited

[shandou]

Took this one for the selfish reason of getting a better understanding of this 😅

[ericmjl]

Thanks @shandou! I'm sure it'll be helpful for others too :).

[hectormz]

@shandou I came across this post which should be pretty helpful from Atlassian:

Git Forks and Upstreams: How-to and a cool tip

I think it might also be helpful to have some details (or link to a resource) on dealing with conflicts when rebasing. That can certainly lead to a newcomer bailing.