[Document Policy] Add feature author guide

This CL adds feature author guide section for document policy
to README.md.

Change-Id: Ib75b203192545fbbb8de48ceef25f1156398c4a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380609Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Charlie Hu <chenleihu@google.com>
Cr-Commit-Position: refs/heads/master@{#805959}

third_party/blink/common/feature_policy/README.md

@@ -68,7 +68,43 @@ To test the new feature with feature policy, refer to
 `third_party/blink/web_tests/external/wpt/feature-policy/README.md` for
 instructions on how to use the feature policy test framework.
 
+## Document Policy Guide
+### How to add a new feature to document policy
+
+Document Policy (see [spec](https://w3c.github.io/webappsec-permissions-policy/document-policy.html)) is a similar mechanism to Feature Policy. It is intended
+to cover those kinds of features which don't involve delegation of permission to trusted origins; features which are more about configuring a document, or removing features (sandboxing) from a document or a frame. Document Policy can only be set through HTTP header, and will not inherit to subframes.
+
+Example HTTP header: `Document-Policy: force-load-at-top=?0, lossy-images-max-bpp=1.0`
+
+- `force-load-at-top` is set to boolean value false (`?0` in [structured header syntax](https://httpwg.org/http-extensions/draft-ietf-httpbis-header-structure.html)), i.e. the
+ feature is disallowed in current document;
+- `lossy-images-max-bpp` is set to 1.0, i.e. lossy image format (e.g. jpeg) images with
+byte per pixel rate higher than 1.0 will be blocked.
+
+
+#### Adding a new feature to document policy
+
+##### Shipping features behind a flag
+If the additional feature is unshipped, or if the correct behaviour with document
+policy is undetermined, consider shipping the feature behind a runtime-enabled feature.
+
+##### Define new feature
+1. Document policy features are defined in
+`third_party/blink/renderer/core/feature_policy/document_policy_features.json5`. Add the new feature,
+placing any runtime-enabled feature or origin trial dependencies in its "depends_on" field as
+described in the file's comments.
+
+2. Append the new feature enum with a brief description as well in
+`third_party/blink/public/mojom/feature_policy/document_policy_feature.mojom`
+
+##### Integrate the feature behaviour with document policy
+The most common way to check if features are enabled is `ExecutionContext::IsFeatureEnabled`.
+
+##### Write web-platform-tests
+Please add new tests to `third_party/blink/web_tests/external/wpt/document-policy/`.
+
 #### Contacts
 For more questions, please feel free to reach out to:
 iclelland@chromium.org
+chenleihu@google.com
 (Emerita: loonybear@)

third_party/blink/public/common/feature_policy/document_policy.h

@@ -27,7 +27,7 @@ namespace blink {
 // which embeds the document.
 //
 // See
-// https://github.com/w3c/webappsec-feature-policy/blob/master/document-policy-explainer.md
+// https://github.com/w3c/webappsec-permissions-policy/blob/master/document-policy-explainer.md
 //
 // Key concepts:
 //