Guide for contributors (#208)

Co-authored-by: Marc Nijdam <[email protected]>

Author: dpezely

README.md

@@ -2,3 +2,17 @@
 ![rust workflow](https://github.com/helium/proto/actions/workflows/rust.yml/badge.svg)
 ![node.js workflow](https://github.com/helium/proto/actions/workflows/node.yml/badge.svg)
 ![erlang workflow](https://github.com/helium/proto/actions/workflows/erlang.yml/badge.svg)
+
+## Contributing
+
+- Avoid `float` in Protobufs because representations differ across hardware architectures
+  + There are many floating point representations from IEEE, plus others
+  + i.e., some range of interior digits are random per float spec
+- Frequency should use `uint32` and should be in Hz
+- rssi or signal is always negative, thus use `sint32` and is in deci-dbm (aka `ddbm`) which is `dbm * 10`
+- snr is signal-to-noise ratio and should be `uint32`
+- Fetch and share time in nanos, then truncate to appropriate granularity as needed
+  + e.g., get from OS in nanos
+- Reject any PR unless units are documented inline within Protobuf definition
+- Document units of fields
+- When exceptions to the above occur, please explain *why* within comments