[docs] Add a couple of paragraphs about swift_feature in testing.md

Quickly explain the feature, its intended usage and a warning about not
longer needing `REQUIRES: asserts` anymore.`

Author: drodriguez

docs/Testing.md

@@ -549,6 +549,27 @@ FIXME: full list.
 * ``XFAIL: linux``: tests that need to be adapted for Linux, for example parts
   that depend on Objective-C interop need to be split out.
 
+#### Features ``REQUIRES: swift_feature_...``
+
+Each of the Swift compiler features defined in `include/swift/Basic/Features.def`
+will get a LLVM Lit feature prefixing `swift_feature_` to the feature name
+automatically. The LLVM Lit features will be available only in those
+configurations where the compiler supports the given feature, and will not be
+available when the compiler does not support the feature. This means that
+standard language features and upcoming features will always be available,
+while experimental features will only be available when the compiler supports
+them.
+
+For every test that uses `--enable-experimental-feature` or
+`--enable-upcoming-feature` add a `REQUIRES: swift_feature_...` for each of the
+used features. The `Misc/verify-swift-feature-testing.test-sh` will check that
+every test with those command line arguments have the necessary `REQUIRES:` and
+fail otherwise.
+
+Do NOT add `REQUIRES: asserts` for experimental features anymore. The correct
+usage of `REQUIRES: swift_feature_...` will take care of testing the feature as
+it evolves from experimental, to upcoming, to language feature.
+
 #### Feature ``REQUIRES: executable_test``
 
 This feature marks an executable test. The test harness makes this feature