Add developer.md

DEVELOPER.md

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+# Developing the plugin
+
+## Getting started
+
+* Install go: `brew install go`
+
+## Developing
+
+* Write tests and production code
+* Update the version in version.go
+* Run tests: `go test`
+* Build the plugin: `make all`
+* Run Fluent Bit with the plugin using the template config file: `FILE_PATH=/usr/local/var/log/test.log API_KEY=(your-api-key) BUFFER_SIZE= MAX_RECORDS= fluent-bit -c ./fluent-bit.conf -e ./out_newrelic.so` 
+* Cause a change that you've configured Fluent Bit to pick up: (`echo "FluentBitTest" >> /usr/local/var/log/test.log`)
+* Look in `https://one.newrelic.com/launcher/logger.log-launcher` for your log message ("FluentBitTest")