Add information on Importing Web Platform Tests

This patch adds information on how to handle the most common scenarios when
importing Web Platform Tests.

Author: stwrt

docs/Infrastructure/ImportWPTTests.md

@@ -0,0 +1,21 @@
+# Import Web Platform Tests
+
+WebKit maintains a separate fork of the Web Platform Tests living in `‌LayoutTests/imported/w3c/web-platform-tests`. When changes are made upstream we need to import them to stay up to date.
+
+## Importing Tests from WPT
+
+```
+Tools/Scripts/import-w3c-tests -t web-platform-tests/folder_to_import_here
+```
+
+When running the script above the latest WPT will be downloaded into WebKitBuild directory. The requested files will be copied over into the WebKit WPT directory.
+
+After importing the tests ensure to run `run-webkit-tests` to generate new expectations. You may need to update the `LayoutTests/TestExpectations` which need to be marked `SKIP` based on `import-w3c-tests` output.
+
+## Import WPT Tests from a local checkout of WPT
+
+If you have the upstream WPT repository locally you can skip redownloading it by running the following below.
+
+```
+Tools/Scripts/import-w3c-tests web-platorm-tests/folder_to_import_here -l -s path_to_web_platform_tests
+```