DOC: How to split a PR

[lithomas1]

Location of the documentation

https://pandas.pydata.org/docs/dev/development/contributing.html#updating-your-pull-request

Documentation problem

It is somewhat common to send in a large pull request for review and have a maintainer ask to split a large pull request up into several precursor pull requests. However, the process of doing so can be somewhat tricky, so it should be mentioned in the contributing docs.

Suggested fix for documentation

Add to the contributing docs how to split up a PR.
Maybe use this as an example. (Am writing this from memory, might have errors)

git branch
# * feature-1
#    master
git checkout master
git checkout -b pre-cursor-to-feature-1
# on pre-cursor-to-feature-1 now
git checkout feature-1 /path/to/first/part/precursor/changes
git commit -m "Pre-cursor for feature-1
# After pre-cursor is merged(on branch feature-1 now)
git branch
# * feature-1
#    master
git pull upstream master
# If there are conflicts and you want to take the version in master.
git checkout --theirs path/to/files

[ivanovmg]

Depending on the commits, you may want to suggest an interactive rebase, or simply reset --hard.