add install section to README

[matthyk]

I just added an install section to the README.md, because I think it's very convenient during development if you just have to copy the install command when you want to use fastify-plugin.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm