Check formatting in CI (#162)

CONTRIBUTING.md

@@ -32,8 +32,7 @@ make all
 #### 5. Adding local (checkout) dependencies for rebar3
 
 Please follow the instructions available at
-https://www.rebar3.org/docs/configuration/dependencies/
-
+<https://www.rebar3.org/docs/configuration/dependencies/>
 
 ```shell
 mkdir _checkouts
@@ -44,7 +43,7 @@ ln -s ../<projectY dependency dir> _checkouts/
 ## Handlers' versioning
 
 As of 1.1.0 release, the custom handlers' implementation has been extended to
-support versioning. register_handler/3 and get_versioned_handlers/0 functions 
+support versioning. register_handler/3 and get_versioned_handlers/0 functions
 have been added. Using versioning in new handlers is encouraged.
 
 ## Testing
@@ -73,6 +72,14 @@ The [dnstest](https://github.com/dnsimple/dnstest) tool provides a suite of blac
 
 Then you will need to run erldns. At this point it should be ready to test with dnstest. See the dnstest README for more details on how to run dnstest.
 
+## Formatting
+
+If your editor doesn't automatically format Erlang code using [erlfmt](https://github.com/WhatsApp/erlfmt), run:
+
+```bash
+make format
+```
+
 ## Releasing
 
 1. Run the test suite and ensure all the tests pass.

Makefile

@@ -26,4 +26,9 @@ clean: $(REBAR)
 .PHONY: test
 test: $(REBAR)
 	$(REBAR) eunit
+	$(REBAR) fmt --check
 	$(REBAR) dialyzer
+
+.PHONY: format
+format: $(REBAR)
+	$(REBAR) fmt