Add note for adding a build tag in new e2e files. (cosmos#4937)

e2e/README.md

@@ -28,6 +28,9 @@ All tests should go under the [e2e](https://github.com/cosmos/ibc-go/tree/main/e
 to an existing test suite ***in the same file***, or create a new test suite in a new file and add test functions there.
 New test files should follow the convention of `module_name_test.go`.
 
+After creating a new test file, be sure to add a build constraint that ensures this file will **not** be included in the package to be built when
+running tests locally via `make test`. For an example of this, see any of the existing test files.
+
 New test suites should be composed of `testsuite.E2ETestSuite`. This type has lots of useful helper functionality that will
 be quite common in most tests.