add formatting guidelines

CONTRIBUTING.md

@@ -9,6 +9,10 @@ our way or a bug to report:
   repo](https://github.com/helium/longfi-arduino). Include a
   write up of the PR with details on what it does.
 
+* **Formatting Code**: Please apply clang-format to any PRs. From the
+  root of the repository: 
+	`find . -regex '.*\.\(cpp\|hpp\|cu\|c\|h\|ino\)' -exec clang-format -i {} \;`
+
 * **Reporting Bugs**: Open an issue [against this
   repo](https://github.com/helium/longfi-arduino/issues) with as much
   detail as you can. At the very least you'll include steps to
@@ -19,4 +23,4 @@ collaboration, and contributors are expected to adhere to the
 [Contributor Covenant Code of
 Conduct](http://contributor-covenant.org/).
 
-Above all, thank you for taking the time to be a part of the Helium community.
+Above all, thank you for taking the time to be a part of the Helium community.