[WPT/common/security-features] Update README.md

- Move generator-related sections from referrer-policy/README.md
  to common/security-features/README.md
- Add mixed-content/README.md to mention about the generator.
- Update some sections, namely how to remove generated tests, etc.

Bug: 906850
Change-Id: I03f5483435dd9a59b923303d693d7a2cf4717e30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785899
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710989}

third_party/blink/web_tests/external/wpt/mixed-content/README.md

+# Mixed-content Web Platform Tests
+
+The subdirectory `gen/` is generated using the generator at `common/security-features`.
+See [common/security-features/README.md](../common/security-features/README.md) for how to generate tests.

third_party/blink/web_tests/external/wpt/upgrade-insecure-requests/README.md

+# Upgrade-insecure-requests Web Platform Tests
+
+The subdirectory `gen/` is generated using the generator at `common/security-features`.
+See [common/security-features/README.md](../common/security-features/README.md) for how to generate tests.