add aerc to the list of mail clients

Thank you for taking the time to work on a PR for Awesome-Sysadmin!

To ensure your PR is dealt with swiftly please check the following:

[x] Your additions are Free software
[x] Software your are submitting is not your own, unless you have a healthy ecosystem with a few contributors (which aren't your sock puppet accounts).
[x] Submit one item per pull request. This eases reviewing and speeds up inclusion.
[x] Format your submission as follows, where Demo and Clients are optional. Do not add a duplicate Source code link if it is the same as the main link. Keep the short description under 80 characters and use sentence case for it, even if the project's webpage or readme uses another capitalisation. Demo links should only be used for interactive demos, i.e. not video demonstrations. - [Name](http://homepage/) - Short description, under 250 characters, sentence case. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language`
[x] Additions are inserted preserving alphabetical order.
[x] Additions are not already listed at awesome-selfhosted
[x] The Language tag is the main server-side requirement for the software. Don't include frameworks or specific dialects.
[x] You have searched the repository for any relevant issues or PRs, including closed ones.
[x] Any category you are creating has the minimum requirement of 3 items.
[x] Any software project you are adding to the list is actively maintained.
[x] The pull request title is informative, unlike "Update README.md". Suggested titles: "Add aaa to bbb" for adding software aaa to section bbb, "Remove aaa from bbb" for removing, "Fix license for aaa", etc.

Please take some time to answer the following questions as best you can:

Why is it awesome?

One of the best mail clients I've used (and contributed to). Fast, and works great with an email-based git-workflow.

Have you used it? For how long?

Yes, for years.

Is this in a personal or professional setup?

both

How many devices/users/services/... do you manage with it?

accessing 3 Mailservers for the personal setup and one for the professional.

Biggest pros/cons compared to other solutions?
- fast startup times allow quick application of patches
- excellent documentation in the form of manpages and a wiki for tips and tricks
- TUIs are hardly "normie"-friendly
- the setup for OAuth is not ideal
Any other comments about your use case, things you've found excellent, limitations you've encountered... ?

awesome-foss / awesome-sysadmin

add aerc to the list of mail clients #546