Add a primitive contributing guide and tweak PR template.

[reivilibre]

As per a discussion with Brendan, I have also tweaked the wording to encourage people to add themselves as 'Contributed by ....', since we generally like to see that but empirically not many people add themselves.

Ameliorates #381 but note that the contributing guide still has a couple of TODO blocks commented out, since this repository doesn't use tox and I don't actually have an example of how to run the tests since I haven't worked with it (I would guess trial is the way, but should be careful not to mislead).

[H-Shay]

For the testing section, would it be possible to just grab the relevant info from the REAME.md doc and add? https://github.com/matrix-org/sydent#testing
This was how I was running the tests.

[clokep]

Seems reasonable overall, I left a couple of comments (and also see @H-Shay's).

[clokep]

I think changing this will let you uncomment this section:

Suggested change

	tox -e py
	python -m twisted.trial tests

[reivilibre]

Thanks, that sounds good.

Thanks to @H-Shay I noticed that the README includes instructions, but says to use trial tests.

I seem to have forgotten the -e flag to pip install -e .[dev] and notice that:

• python -m twisted.trial tests works
• but trial tests fails

(I only just realised that I probably forgot -e and that might be the cause.)

I never really appreciated the difference there, but I guess python -m treats all subdirs of the current working dir as modules, whereas trial tests expects the tests modules to be installed.

I guess I will leave a hint about that, since this tripped me up for a few minutes and I suspect it could be confusing to someone not used to Python's modules.

[clokep]

How is trial tests failing? Usually this is a path issue (it is finding the wrong trial binary). Using python -m twisted.trial should generally be the same... 🤷

[reivilibre]

At first, it failed saying that it can't find tests.

Then I did pip install .[dev] and it now finds the tests but can't find the templates.

Only with pip install -e .[dev] does trial work.

I think this is because invoking trial will only use the modules available in your venv, but python -m twisted.trial seems to have the current directory on Python's path? I'm not ultimately that sure either.