Rewrite installation and invocation instructions using Composer

[Xymph]

This PR was prompted by this observation. Some of the text was copied from the AddWiki docs, I hope that is alright.

@samwilson, is this what you had in mind?

[waldyrious]

Nice! I left a couple suggestions inline, let me know what you think.

[waldyrious]

LGTM :) Let's see what @samwilson has to say.

[samwilson]

Looks good to me, reads well. I only have a tiny comment about the version number, and I was going to say that something should be added to the release process — but it's already mentioned in GOVERNANCE.md. :-)

[samwilson]

It doesn't matter massively before 1.0, but I'd suggest writing this as the slightly more lenient ^0.15. This is because (once the trailing zero becomes a 1 or whatever), if a project depends on two libraries that both depend on Wikimate, there's less chance of version conflicts. This doesn't really matter before 1.0, and anyway it's probably not that likely for multiple libraries to depend on Wikimate, so feel free to ignore this comment! :-)

[Xymph]

It doesn't matter massively before 1.0, but I'd suggest writing this as the slightly more lenient ^0.15. This is because (once the trailing zero becomes a 1 or whatever), if a project depends on two libraries that both depend on Wikimate, there's less chance of version conflicts. This doesn't really matter before 1.0, and anyway it's probably not that likely for multiple libraries to depend on Wikimate, so feel free to ignore this comment! :-)

Thanks, I made the change.
But I am sort of wondering if what you wrote applies more to ~1.0 rather than ^1.0 (1.0.0 will indeed be our next release).

Per the documentation my understanding is that ^1.0.0 will allow ">=1.0.0 <2.0.0" and is thus very lenient, while ~1.0.0 allows only ">1.0.0 <1.1.0" and thus indeed increases the risk of a version conflict (so ~1.0 would be better indeed). But I'm new to this, so open to enlightenment. :)

[Xymph]

The prerelease example PR #81 in GOVERNANCE.md is a bit old, includes some discussion clutter, and doesn't show how to propose a Release Summary. Hence the change to short-and-clean #126.

[waldyrious]

I'll merge in a bit, but first will change the title of the PR to be in the imperative mood, like the commit messages (and unlike the CHANGELOG.md file).