Documentation: add nix cache instructions

[isovector]

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[stephen-smith]

There's already docs for this in docs/src/how-to/install/dependencies.rst (listed as "Optional") and docs/src/developer/developer/dependencies.md (in the configuration), but I don't see much disadvantage to mentioning it here, as well.

[isovector]

Is that document linked from the main README? Maybe I missed it?

[stephen-smith]

The dependencies.rst isn't linked from anywhere I can see. The dependencies.md is linked from how-to.md in the same directory, but how-to.md isn't linked from anywhere. So, I do not see a link chain from README.md to either of those locations.

[sysvinit]

We have some internal documentation for Nix and Cachix setup in the README for cailleach (which I don't think @isovector is able to see). I think this might have come up once or twice in the past, but as the instructions are relevant both for wire-server and cailleach, it might be worth moving these docs into the public repo so they're visible to external contributors.