Track policies (with a view to create POLICIES.md)

[sjwarner-bp]

It would be most useful to have a succinct list of the current track policies, to allow future contributors (and reviewers) to facilitate the most desirable contributions.

This would also be a great reference tool, and would be a useful part of a template for PRs.

Before this PR, I'd like a discussion to take place regarding current policies (older members of @exercism/bash may have a better idea of longer-standing policies) and policies we should strive to implement/achieve.

[sjwarner-bp]

It seems like a good starting point would be "Plain Bash over POSIX compliance", as decided in #88.

[sjwarner-bp]

Another could be to formally record an onboarding process / document how we will ensure that onboarding is as easy as possible.

The Java track (where I have most experience) do this by having a number of good-first-patch issues open at any one time.

[sjwarner-bp]

Further, documenting the decision to come out of #87 would be beneficial to future contributors.

[budmc29]

POSIX compliance can be added: #88 (comment)

[budmc29]

Test structure can be added: #150