Exporter-only build+install docs

[matthudsonatx]

Types of changes

This documents the build/deploy process that worked for me. I put Grafana and Prometheus on a Raspberry Pi, and built the exporter for local distribution using this process.

I'm not sure if anyone needs this but here it is, just in case!

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• [x ] I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• [ x] I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.