Markdown-ify README, add logo

Summary: Makes the README markdown formatted and adds the logo on the top. :)

Reviewed By: siyengar

Differential Revision: D8465425

fbshipit-source-id: d15b4e7848f729da569f56e90b0f18e9bd461bc4

README.md

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+<p align="center">
+  <img width="500" height="216" alt="Fizz" src="https://raw.github.com/facebookincubator/fizz/master/logo2x.png">
+</p>
+Fizz is a TLS 1.3 implementation.
+
+Fizz currently supports TLS 1.3 drafts 18-28. HelloRetryRequest, early data,
+and client authentication are only supported on draft 19 and above. Each draft
+also has a corresponding "fb" version. These match the draft versions except
+for some minor record layer differences.
+
+## Dependencies
+Fizz largely depends on three libraries: [folly](https://www.github.com/facebook/folly),
+[OpenSSL](https://www.openssl.org/), and [libsodium](https://github.com/jedisct1/libsodium).
+
+## Source Layout
+- `fizz/crypto`:   Cryptographic primitive implementations (most are wrapping
+                   OpenSSL or libsodium)
+- `fizz/record`:   TLS 1.3 record layer parsing
+- `fizz/protocol`: Common protocol code shared between client and server
+- `fizz/client`:   Client protocol implementation
+- `fizz/server`:   Server protocol implementation
+
+## Design
+The core protocol implementations are in `ClientProtocol` and `ServerProtocol`.
+`FizzClientContext` and `FizzServerContext` provide configuration options.
+`FizzClient` and `FizzServer` (which both inherit from `FizzBase`) provide
+applications with an interface to interact with the state machine.
+`FizzClient`/`FizzServer` receives events from the application layer, invokes the
+correct event handler, and invokes the application `ActionVisitor` to process the
+actions.
+
+`AsyncFizzClient` and `AsyncFizzServer` provide implementations of the folly
+`AsyncTransportWrapper` interface. They own an underlying transport (for example
+`AsyncSocket`) and perform the TLS handshake and encrypt/decrypt application
+data.
+
+## Sample Applications
+`ClientSocket` and `ServerSocket` provide sample usage of `AsyncFizzClient` and
+`AsyncFizzServer` and can be used to start up a simple TLS 1.3 client or server
+over a TCP connection.
+
+For example, to start ServerSocket on port 443 with a specified cert:
+`ServerSocket -port 443 -cert foo.pem -key foo.key`. Then, on the same host,
+you can connect with `ClientSocket -host localhost -port 443`. Both will dump
+the data they get and will remain running until interrupted via CTRL+C.