primitives: Add skeleton README.md.

internal/staging/primitives/README.md

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+primitives
+==========
+
+[![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/internal/staging/primitives)
+
+## Package and Module Status
+
+This package is currently a work in progress in the context of a larger refactor
+and thus does not yet provide most of the things that are ultimately planned.
+See https://github.com/decred/dcrd/issues/2786 for further details.
+
+The intention is to create a containing `primitives` module that will be kept at
+an experimental module version ("v0") until everything is stabilized to avoid
+major module version churn in the mean time.
+
+## Overview
+
+This package ultimately aims to provide core data structures and functions for
+working with several aspects of Decred consensus.
+
+The provided functions fall into the following categories:
+
+- Proof-of-work
+  - Converting to and from the target difficulty bits representation
+  - Calculating work values based on the target difficulty bits
+  - Checking that a block hash satisfies a target difficulty and that the target
+    difficulty is within a valid range
+
+## Maintainer Note
+
+Since the `primitives` module is heavily relied upon by consensus code, there
+are some important aspects that must be kept in mind when working on this code:
+
+- It must provide correctness guarantees and full test coverage
+- Be extremely careful when making any changes to avoid breaking consensus
+  - This often means existing code can't be changed without adding an internal
+    flag to control behavior and introducing a new method with the new behavior
+- Minimize the number of allocations as much as possible
+- Opt for data structures that improve cache locality
+- Keep a strong focus on providing efficient code
+- Avoid external dependencies
+  - Consensus code requires much stronger guarantees than typical code and
+    consequently code that is not specifically designed with such rigid
+    constraints will almost invariably inevitably break things in subtle ways
+    over time
+- Do not change the API in a way that requires a new major semantic version
+  - This is not entirely prohibited, but major module version bumps have severe
+    ramifications on every consumer, and thus they should be an absolute last
+    resort
+- Take care when adding new methods to avoid method signatures that would
+  require a major version bump due to a new major dependency version
+
+## Installation and Updating
+
+This package is internal and therefore is neither directly installed nor needs
+to be manually updated.
+
+## License
+
+Package primitives is licensed under the [copyfree](http://copyfree.org) ISC
+License.