DOC: How to split a PR
#### 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
```
