docs: ActivityPub層の変更を含む場合にやるべきことを明文化

CONTRIBUTING.md

@@ -64,6 +64,22 @@ Thank you for your PR! Before creating a PR, please check the following:
 
 Thanks for your cooperation 🤗
 
+### Additional things for ActivityPub payload changes
+*This section is specific to misskey-dev implementation. Other fork or implementation may take different way. A significant difference is that non-"misskey-dev" extension is not described in the misskey-hub's document.*
+
+If PR includes changes to ActivityPub payload, please reflect it in [misskey-hub's document](https://github.com/misskey-dev/misskey-hub-next/blob/master/content/ns.md) by sending PR.
+
+The name of purporsed extension property (referred as "extended property" in later) to ActivityPub shall be prefixed by `_misskey_`. (i.e. `_misskey_quote`)
+
+The extended property in `packages/backend/src/core/activitypub/type.ts` **must** be declared as optional because ActivityPub payloads that comes from older Misskey or other implementation may not contain it.
+
+The extended property must be included in the context definition. Context is defined in `packages/backend/src/core/activitypub/misc/contexts.ts`.
+The key shall be same as the name of extended property, and the value shall be same as "short IRI".
+
+"Short IRI" is defined in misskey-hub's document, but usually takes form of `misskey:<name of extended property>`. (i.e. `misskey:_misskey_quote`)
+
+One should not add property that has defined before by other implementation, or add custom variant value to "well-known" property.
+
 ## Reviewers guide
 Be willing to comment on the good points and not just the things you want fixed 💯