Add new README and API overview pages.

Signed-off-by: Michael Warres <mpw@google.com>

README.md

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+# Proxy-Wasm C++ SDK
+
+[![Build Status][build-badge]][build-link]
+[![Apache 2.0 License][license-badge]][license-link]
+
+Proxy-Wasm is a specification and supporting framework for using
+[WebAssembly](https://webassembly.org) (Wasm) to extend the functionality of
+network proxies. It enables developers to write custom logic (plugins) that are
+compiled to Wasm modules and then loaded and executed by the proxy.
+
+Proxy-Wasm consists of multiple parts:
+
+* An [ABI](https://github.com/proxy-wasm/spec) that specifies the low-level
+  interface between network proxies and Wasm virtual machines that run the
+  plugins.
+* [Host implementations](https://github.com/proxy-wasm/spec#host-environments)
+  of the ABI, provided by network proxies.
+* [Language-specific SDKs](https://github.com/proxy-wasm/spec#sdks) that layer
+  on top of the ABI, providing a more natural and programmer-friendly API for
+  invoking and implementing Proxy-Wasm functions and callbacks.
+
+This repository provides the C++ SDK.
+
+## Getting started
+
+* Read the [API overview](docs/api_overview.md) to learn about [Proxy-Wasm
+  concepts] and how they are represented in the C++ SDK.
+* View an [example plugin](example/http_wasm_example.cc).
+* Refer to [API documentation](docs/api_overview.md#codemap).
+* [Build](docs/building.md) plugin code.
+
+[build-badge]: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/workflows/C++/badge.svg?branch=master
+[build-link]: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/actions?query=workflow%3AC%2B%2B+branch%3Amaster
+[license-badge]: https://img.shields.io/github/license/proxy-wasm/proxy-wasm-cpp-sdk
+[license-link]: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/blob/master/LICENSE