docs: add protocols description

docs/src/content/concepts-protocols.md

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+---
+title: "Protocols"
+menu:
+    concepts:
+        weight: 7
+---
+
+# Protocols
+
+## HTTP/1.0 and HTTP/1.1
+
+[RFC7230: HTTP/1.1: Message Syntax and Routing](http://tools.ietf.org/html/rfc7230)
+
+[RFC7231: HTTP/1.1: Semantics and Content](http://tools.ietf.org/html/rfc7231)
+
+HTTP/1.0 and HTTP/1.1 support in mitmproxy is based on our custom HTTP stack,
+which takes care of all semantics and on-the-wire parsing/serialization tasks.
+
+mitmproxy currently does not support HTTP trailers - but if you want to send
+us a PR, we promise to take look!
+
+## HTTP/2
+
+[RFC7540: Hypertext Transfer Protocol Version 2 (HTTP/2)](http://tools.ietf.org/html/rfc7540>)
+
+HTTP/2 support in mitmproxy is based on
+[hyper-h2](https://github.com/python-hyper/hyper-h2). It fully encapsulates the
+internal state of HTTP/2 connections and provides an easy-to-use event-based
+API. mitmproxy supports the majority of HTTP/2 feature and tries to
+transparently pass-through as much information as possible.
+
+mitmproxy currently does not support HTTP/2 trailers - but if you want to send
+us a PR, we promise to take look!
+
+mitmproxy currently does not support HTTP/2 Cleartext (h2c) since none of the
+major browser vendors have implemented it.
+
+Some websites are still having problems with correct HTTP/2 support in their
+webservers and can cause errors, dropped connectiones, or simply no response at
+all. We are trying to be as tolerant and forgiving as possible with the types of
+data we send and receive, but
+[some](https://github.com/mitmproxy/mitmproxy/issues/1745)
+[faulty](https://github.com/mitmproxy/mitmproxy/issues/2823)
+[implementations](https://github.com/mitmproxy/mitmproxy/issues/1824)
+[simply](https://github.com/mitmproxy/mitmproxy/issues/1891) don't work well
+with mitmproxy.
+
+In order to increase the compatibility of mitmproxy with HTTP/2 webservers, we
+default to NOT forward any priority information that is sent by a client. You
+can enable it with: `http2_priority=true`.
+
+## WebSocket
+
+[RFC6455: The WebSocket Protocol](http://tools.ietf.org/html/rfc6455)
+
+[RFC7692: Compression Extensions for WebSocket](http://tools.ietf.org/html/rfc7692)
+
+WebSocket support in mitmproxy is based on [wsproto]
+(https://github.com/python-hyper/wsproto) project. It fully encapsulates
+WebSocket frames/messages/connections and provides an easy-to-use event-based
+API.
+
+mitmproxy fully supports the compression extension for WebSocket messages,
+provided by wsproto. Message contents are automatically compressed and
+decompressed before firing events.
+
+mitmproxy currently does not display WebSocket messages in the console or web
+UI. Only the WebSocket handshake flow is shown, which contains a reference to
+the parent flow for all messages exchanged over this connection.
+
+If an endpoint sends a PING to mitmproxy, a PONG will be sent back immediately
+(with the same payload if present). To keep the other connection alive, a new
+PING (without a payload) is sent to the other endpoint. Unsolicited PONG's are
+not forwarded. All PING's and PONG's are logged (with payload if present).
+
+## Raw TCP / TCP Proxy / Fallback
+
+In case mitmproxy does not handle a specific protocol, you can exempt
+hostnames from processing, so that mitmproxy acts as a generic TCP forwarder.
+This feature is closely related to the *passthrough* functionality,
+but differs in two important aspects:
+
+  * The raw TCP messages are printed to the event log.
+  * SSL connections will be intercepted.
+
+Please note that message interception or modification are not possible yet. If
+you are not interested in the raw TCP messages, you should use the ignore
+domains feature.
+
+|                    |                    |
+| ------------------ | ------------------ |
+| command-line alias | `--tcp HOST`       |
+| mitmproxy shortcut | press `O` then `T` |