chore(docs): add developing instructions

DEVELOPING.md

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+# Developing
+
+## Testing pact_broker-client locally
+
+1. Make sure you have `ruby` available on the command line
+2. Install with `bundle install`
+3. Run tests with `bundle exec rake`
+4. Run individual test with `rspec <path to test>`
+5. Run one of the scripts in `./example/scripts`, see the readme for more detail, and scripts
+
+##  Available Rake tasks
+
+1. check `bundle exec rake --tasks`
+
+## Generating docs
+
+If you change the CLI commands, run `bundle exec script/update-cli-usage-in-readme.rb` to update the `README.md` automatically.