README.md

### Folder Structure:

* **opt_32**
  * Optimized 32-bit implementation in C.
* **opt_32_armv67**
  * Optimized 32-bit implementation in ARMv6/ARMv7 assembly.
* **opt_64**
  * Optimized 64-bit implementation in C.
* **ref**
  * Easy-to-read implementation in C.