Add cherry-pick guide for fetching new material from upstream

[egrace479]

As this template is updated, we pull new material into both the Imageomics and ABC Guides. Anyone else using this template may also wish to pull updates. Other than the documentation, there's not much for available guidance on cherry picking commits, so this PR adds a git cherry-pick guide, specifically using this repo as the example source.

I also added a link to this section from the "How to Use the Guide" section of the repo README.

[NetZissou]

Nice. Definitely need this!

[thompsonmj]

Definitely helpful, though in this presentation, feels a bit 'meta'—like it maybe belongs in a CONTRIBUTING.md file rather than a seamless entry in the guide itself.

Perhaps it could be framed in a more generic way by frontloading with some example use-cases for what cherry picking does and when you might want to use it? Then offering the task of updating this repo itself as an example? Also linking to this page from the corresponding section under the Two Repo Problem.

[egrace479]

Definitely helpful, though in this presentation, feels a bit 'meta'—like it maybe belongs in a CONTRIBUTING.md file rather than a seamless entry in the guide itself.

Perhaps it could be framed in a more generic way by frontloading with some example use-cases for what cherry picking does and when you might want to use it? Then offering the task of updating this repo itself as an example? Also linking to this page from the corresponding section under the Two Repo Problem.

I see your point. It's hard to motivate, as git cherry-pick is basically just pulling a commit from elsewhere, which I haven't done outside this and the Two-Repo Problem case. I'm open to suggestions to give it a bit more of an introduction though.

I would argue that it doesn't belong in CONTRIBUTING.md though, since it's not sending changes back here, but pulling the changes made in the upstream repo. I certainly wouldn't want to set an expectation that one should contribute if they want to use the template. It was definitely made with the motivation to help end users though.

[egrace479]

I feel like we should add a link from the two repo cherry-pick section back to this page too. @thompsonmj, what do you think?

[thompsonmj]

Added a small suggestion, but looks good!