CONTRIBUTING: Add documentation for how to add a test FIT file. (#85)

CONTRIBUTING.md

@@ -12,3 +12,24 @@ when filing an issue.
 
 Please follow standard Go conventions and make sure that invoking ```make``` in
 the root directory reports no errors.
+
+## Adding test FIT files
+
+When contributing a change to the parsing of FIT files it is beneficial to
+include a FIT file which causes the issue you have resolved. To ensure the
+FIT file is executed during tests you must do the following:
+
+Include the FIT file in an appropriate subdirectory of "testdata" such as
+"misc".
+
+Add an entry to decodeTestFiles in the file reader_files_test.go, ensuring
+that the fingerprint field is set to "1".
+
+Execute ```go test -update``` to calculate the fingerprint and update it. This
+will also produce a gzipped golden test output file in the same directory as
+your original file. This file can be inspected by using a tool like ```zcat```.
+
+Execute ```make test``` and ensure all tests have passed.
+
+Commit your newly added FIT file, the generated gzipped golden test output
+file, and the reader_files_test.go file.