blake256: Add internal _asm README.md.

crypto/blake256/internal/_asm/README.md

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+_asm
+====
+
+[![Build Status](https://github.com/decred/dcrd/workflows/Build%20and%20Test/badge.svg)](https://github.com/decred/dcrd/actions)
+[![ISC License](https://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
+[![Doc](https://img.shields.io/badge/doc-reference-blue.svg)](https://pkg.go.dev/github.com/decred/dcrd/crypto/blake256/internal/_asm)
+
+## Overview
+
+This provides a separate internal module for generating the assembly code that
+provides vector acceleration for the BLAKE-224 and BLAKE-256 block compression
+function.
+
+It uses Go to generate the assembly functions and related stubs to support SSE2,
+SSE4.1, and AVX for `amd64` via [avo](https://github.com/mmcloughlin/avo).
+
+The internal module ensures the specific version of avo used to generate the
+code is pinned and therefore entirely reproducible without adding an otherwise
+unnecessary dependency to the main public module.
+
+## Generating Code
+
+The assembly code can be generating by running the following command in this
+directory:
+
+```bash
+$ go generate
+```
+
+## Installation and Updating
+
+This package is internal and therefore is neither directly installed nor needs
+to be manually updated.
+
+## License
+
+Package _asm is licensed under the [copyfree](http://copyfree.org) ISC License.