feat: doc build deps and build process

README.md

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+[![status-badge](https://ci.batsense.net/api/badges/109/status.svg)](https://ci.batsense.net/repos/109)
+
+---
+
+# gomcaptcha: Go library that implements mCaptcha PoW
+
+## Overview
+
+Normal usage within mCaptcha ecosystem will look like this:
+
+1. mCaptcha server presents challenge parameters (`difficulty_factor`, salt, and challenge phrase)
+2. Visitor computes Proof-of-Work using parameters received
+3. Visitor submits Proof-of-Work to mCaptcha server and receives
+   authorization token if Proof-of-Work is valid.
+4. Visitor submits Proof-of-Work to protected resource/server
+
+This library implements step #2 from this process.
+
+## Dependencies
+
+1. [Rust compiler](https://rustup.rs): This library is a thin wrapper
+   over [mcaptcha-pow-ffi](./mcaptcha-pow-ffi), which has to be compiled
+   first before this library can be used.
+2. Go
+3. GNU Make
+
+Compile Rust FFI library:
+
+```bash
+make rust.build
+```
+
+Run example:
+
+```bash
+make example
+```
+
+## Usage
+
+Please refer to [example](./examples/main.go).