This repo is missing a README

[Mr0grog]

As part of ipfs-inactive/docs#55, we’re trying to make sure every repo has a useful readme and description. go-metrics-prometheus has a description, but no README at all, so we should make sure we add one.

It doesn’t need to be too special, but we should at least explain:

• Name
• More detailed description that the GitHub description field (even if it’s only one short paragraph). Try and say how it fits into IPFS (e.g. go-ipfs relies on this module to send metrics when you configure xyz).
• How to install (even if just go get xyz or git clone xyz)
• Standard license text — you can pretty much just copy & paste.
• Bonus points for a usage example if it’s easy to whip one up!