feat: update README.md with recent info about AP

Signed-off-by: Evan Prodromou <github@evanprodromou.name>

README.md

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+# ActivityPub
+
+[ActivityPub](https://www.w3.org/TR/activitypub/) is the decentralized social networking protocol.
+
+It provides a client to server API for creating, updating and deleting content, mechanisms for building a social graph, as well as a federated server to server API for delivering notifications and subscribing to content.
+
+It is based on the [ActivityStreams 2.0](https://www.w3.org/TR/activitystreams-core/) (AS2) data format, which is a JSON-LD format for describing social activities. Because AS2 is extensible with new types of activities, objects, and properties, ActivityPub is also extensible. You can build many different kinds of social applications on top of ActivityPub.
+
+ActivityPub was developed by the [Social Web Working Group](https://www.w3.org/Social/WG) of the [World Wide Web Consortium](https://www.w3.org/). It was standardized in 2018 as a [W3C Recommendation](https://www.w3.org/TR/activitypub/).
+
+## Key info
+
+- The [ActivityPub specification](https://www.w3.org/TR/activitypub/) is the official document that describes the protocol.
+- The [errata](https://www.w3.org/wiki/ActivityPub_errata) pages shows known errors in the specification.
+- The [Editor's Draft](https://w3.github.io/activitypub/) incorporates the corrected errata directly into the specification text.
+- The [SocialCG](https://www.w3.org/community/socialcg/) is the community group that maintains the specification. It meets regularly to discuss the specification and its implementations.
+- The [ActivityPub Primer](https://www.w3.org/wiki/ActivityPub/Primer) gives deeper explanations of topics described in the specification.
+- The [ActivityPub WebFinger profile](https://swicg.github.io/activitypub-webfinger/) describes how to use WebFinger with ActivityPub.
+- The [ActivityPub HTTP Signatures profile](https://swicg.github.io/activitypub-http-signatures/) describes how to use HTTP Signatures with ActivityPub.
+- The [ActivityPub Data Portability task force](https://swicg.github.io/activitypub-data-portability/) describes how to use ActivityPub for data portability.
+
+## Contributions
+
+The main way to make contributions, ask questions, or report errors is to make a [GitHub issue](https://github.com/w3c/activitypub/issues). Because the specification is a published W3C Recommendation, it is *not* helpful to make pull requests to the repository.
+
+## Processes
+
+Because the document is a published W3C Recommendation, the process for making changes to the specification is more formal than for other documents.
+
+### Clarifying practices
+
+Explanations, tips and clarifications usually go in the [ActivityPub Primer](https://www.w3.org/wiki/ActivityPub/Primer). If you have a question about how something works in ActivityPub, open an issue and we can discuss adding it to the primer.
+
+### Correcting errors
+
+To handle editorial errors like spelling or grammar mistakes, unclear or ambiguous text, factual errors in text, or syntax errors in examples, we have several steps.
+
+1. Make a GitHub issue.
+2. The editor will make a [proposed erratum](https://www.w3.org/wiki/ActivityPub_errata/Proposed) for review by the SocialCG.
+3. At a future SocialCG meeting, the group will review the proposed erratum and decide whether to accept it. Accepted errata are added to the [errata](https://www.w3.org/wiki/ActivityPub_errata) page.
+4. The editor will incorporate the errata into the [Editor's Draft](https://w3.github.io/activitypub/).
+5. Errata are periodically deployed to the main [ActivityPub specification](https://www.w3.org/TR/activitypub/).
+
+### Backwards-compatible changes
+
+Backwards-compatible changes to ActivityPub include the following:
+
+- Adding new kinds of activities
+- Adding new kinds of objects
+- Making mandatory behaviours optional
+- Adding new properties to existing types
+
+Backwards-compatible changes should usually be implemented as extensions to the specification. The [Activity Streams 2.0 primer](https://www.w3.org/wiki/Activity_Streams/Primer/Extensions) describes the extension architecture for ActivityStreams 2.0. The [Extensions Policy](https://swicg.github.io/extensions-policy/) describes the process for incorporating popular extensions into the main Activity Streams 2.0 context document.
+
+Many extensions to ActivityPub are proposed and discussed as part of the [Fediverse Enhancement Proposals](https://codeberg.org/fediverse/fep) process. This process is not formally part of the W3C, but it is a useful way to discuss and develop extensions to ActivityPub.
+
+### Non-backwards-compatible changes
+
+Non-backwards-compatible changes to the specification require chartering a new working group at the W3C. It also requires making changes in dozens of ActivityPub implementations and tens of thousands of running servers. This is a lot of work and is not done lightly.
+
+Examples of such changes:
+
+- Making optional features mandatory
+- Forbidding optional features
+- Deprecating features
+- Making
+
+To propose a breaking change to ActivityPub, add a new issue. It will be discussed and flagged for the next version of ActivityPub.