docs: Add documentation for creating and using yamlfmt's integration tests.

Author: octo

docs/integration-test.md

@@ -0,0 +1,36 @@
+# Integration Testing
+
+yamlfmt has integration testing that tests invocations end-to-end.
+
+## Run
+
+Integration tests are excluded from `go test` by default using build tags. To build and execute the integration test, use:
+
+```shell
+make integrationtest
+```
+
+## Update
+
+The "golden" / expected output files can be (re-)generated by the test framework. To do so, run:
+
+```shell
+make integrationtest_update
+```
+
+## Add new test case
+
+* Run the following command to create the directory structure for the new test:
+
+  ```shell
+  TESTNAME=my_new_test make command_test_case
+  ```
+* Add example input files to `testdata/${TESTNAME}/before`.
+* Add the expected output to `testdata/${TESTNAME}/stdout/stdout.txt` if applicable.
+* `make command_test_case` added a new Go test to `integrationtest/command/command_test.go`. Update the `yamlfmt` flags so the invokation uses the feature the test should cover.
+* Run the following command to generate the expected "golden" output of the test:
+
+  ```shell
+  make integrationtest_update
+  ```
+* Review the generated output to ensure it is expected.