add vendoring

vendor/maunium.net/go/mautrix/README.md

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+# mautrix-go
+[![GoDoc](https://pkg.go.dev/badge/maunium.net/go/mautrix)](https://pkg.go.dev/maunium.net/go/mautrix)
+
+A Golang Matrix framework. Used by [gomuks](https://matrix.org/docs/projects/client/gomuks),
+[go-neb](https://github.com/matrix-org/go-neb), [mautrix-whatsapp](https://github.com/mautrix/whatsapp)
+and others.
+
+Matrix room: [`#maunium:maunium.net`](https://matrix.to/#/#maunium:maunium.net)
+
+This project is based on [matrix-org/gomatrix](https://github.com/matrix-org/gomatrix).
+The original project is licensed under [Apache 2.0](https://github.com/matrix-org/gomatrix/blob/master/LICENSE).
+
+In addition to the basic client API features the original project has, this framework also has:
+
+* Appservice support (Intent API like mautrix-python, room state storage, etc)
+* End-to-end encryption support (incl. interactive SAS verification)
+* Structs for parsing event content
+* Helpers for parsing and generating Matrix HTML
+* Helpers for handling push rules
+
+This project contains modules that are licensed under Apache 2.0:
+
+* [maunium.net/go/mautrix/crypto/canonicaljson](crypto/canonicaljson)
+* [maunium.net/go/mautrix/crypto/olm](crypto/olm)