Add SemVer compatibility badge to README

[greysteil]

Would you folks be up for showing your SemVer compatibility in a badge on your readme? We've just added the ability to do that using data from the test-runs of all the pull requests Dependabot creates - what do you reckon?

Here's what the badge will look like, and where it will link to:

Any feedback super appreciated!

[greysteil]

@nickcampbell18 - any thoughts?

[greysteil]

Updated the text of the badge to semver stability, which should make the use case clearer. Any thoughts?

[nickcampbell18]

No strong feelings. @hmac, thoughts?

[hmac]

Yeah, why not? I assume the number is rounded to the nearest 10%?

[greysteil]

It’s exact - just happens that statesman is at a round number at the moment

[greysteil]

@nickcampbell18 - updated!

[hmac]

Dependabot has updated statesman between SemVer compatible versions 39 times across 12 projects so far. 90% of those updates passed CI.

So 35.1 CI passes?

[greysteil]

OK, so it's rounded to the nearest 1%. You know what I mean @hmac 😜

[petehamilton]

@nickcampbell18 @hmac any idea what's going on with CI? They're all just pending but I can't find anything in CircleCI 🤔

This isn't going to break tests, so I'm just going to merge this in 🚢