added note about the GUFE package

[mikemhenry]

fixes #306

Checklist

• [ ] Added a news entry

Developers certificate of origin

• [ ] I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[mikemhenry]

I put the note in two spots, both seem like the "right" one and duplicating isn't too bad (I can make a macro if we want to put it everywhere there is a GUFE object/want to put it in more spots)

[mikemhenry]

@richardjgowers I was going to do a "for example, and " but that might be unnecessary?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.32%. Comparing base (5304d7c) to head (e6b3d61).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #828 +/- ## ========================================== - Coverage 94.13% 92.32% -1.81% ========================================== Files 134 134 Lines 9786 9786 ========================================== - Hits 9212 9035 -177 - Misses 574 751 +177 ``` | [Flag](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/828/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | Coverage Δ | | |---|---|---| | [fast-tests](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/828/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | `92.32% <ø> (?)` | | | [slow-tests](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/828/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy#carryforward-flags-in-the-pull-request-comment) to find out more.

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[richardjgowers]

@mikemhenry I added an extra sentence to explain a little which people should consult. If you're happy with it merge this and/or copy it into both references. A macro seems excessive for two usages

[mikemhenry]

I will copy it in both spots and merge it in o7