Document supported matrix versions

[ShadowJonathan]

• Document the matrix versions supported ("Compatible with matrix vX" badges/shields#7245) in README
• Make sure /versions endpoint returns this information as well. Covered by Matrix 1.1 checklist #11079

Related to: element-hq/element-web#19674

[reivilibre]

Adding a badge to the README is probably not up to me to say whether we want that or not (I see no harm, I guess).

Adding the information to /versions is already a mentioned task in #11079 so in that sense it's a duplicate.

[anoadragon453]

I also see no harm, and do see the benefit. It's also a nice way to advertise that a project uses Matrix I suppose.

It will be interesting to see if adding these badges catch on on other, non matrix-org/vector-im repos.

[ShadowJonathan]

I've filed a fewlot of issues for this all-around, i think in general it will be an obvious positive all-around for the ecosystem, so i think it'll catch on eventually.