multi: remove all code point to btcd in README

After [this PR](btcsuite/btcd#1785) this repo is
now deprecated as the code lives in `btcd` within a new sub-module. In
this commit, we remove all the code, and update the README to point to
the new location.

README.md

@@ -1,49 +1,4 @@
-btcutil
-=======
-
-[![Build Status](https://github.com/btcsuite/btcutil/workflows/Build%20and%20Test/badge.svg)](https://github.com/btcsuite/btcutil/actions)
-[![ISC License](https://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
-[![GoDoc](https://img.shields.io/badge/godoc-reference-blue.svg)](https://godoc.org/github.com/btcsuite/btcutil)
-
-Package btcutil provides bitcoin-specific convenience functions and types.
-A comprehensive suite of tests is provided to ensure proper functionality.  See
-`test_coverage.txt` for the gocov coverage report.  Alternatively, if you are
-running a POSIX OS, you can run the `cov_report.sh` script for a real-time
-report.
-
-This package was developed for btcd, an alternative full-node implementation of
-bitcoin which is under active development by Conformal.  Although it was
-primarily written for btcd, this package has intentionally been designed so it
-can be used as a standalone package for any projects needing the functionality
-provided.
-
-## Installation and Updating
-
-```bash
-$ go get -u github.com/btcsuite/btcutil
-```
-
-## GPG Verification Key
-
-All official release tags are signed by Conformal so users can ensure the code
-has not been tampered with and is coming from the btcsuite developers.  To
-verify the signature perform the following:
-
-- Download the public key from the Conformal website at
-  https://opensource.conformal.com/GIT-GPG-KEY-conformal.txt
-
-- Import the public key into your GPG keyring:
-  ```bash
-  gpg --import GIT-GPG-KEY-conformal.txt
-  ```
-
-- Verify the release tag with the following command where `TAG_NAME` is a
-  placeholder for the specific tag:
-  ```bash
-  git tag -v TAG_NAME
-  ```
-
-## License
-
-Package btcutil is licensed under the [copyfree](http://copyfree.org) ISC
-License.
+This package is now a sub-package in the
+[btcutil](https://github.com/btcsuite/btcd/tree/master/btcutil) directory
+of the [btcd](https://github.com/btcsuite/btcd) repository.
+