docs: Update for mixing module.

This updates the docs/README.md file, module hierarchy graphviz, and
module hierarchy diagram to reflect the new mixing module.

docs/README.md

@@ -236,6 +236,8 @@ The following versioned modules are provided by dcrd repository:
 * [math/uint256](https://github.com/decred/dcrd/tree/master/math/uint256) -
   Implements highly optimized fixed precision unsigned 256-bit integer
   arithmetic
+* [mixing](https://github.com/decred/dcrd/tree/master/mixing) - Provides
+  infrastructure for Decred's StakeShuffle decentralized mixing
 
 <a name="ModuleHierarchy" />
 

docs/assets/module_hierarchy.gv

@@ -16,6 +16,7 @@ digraph {
 	edwards [label="dcrec/edwards/v2" fillcolor=lightseagreen]
 	secp256k1 [label="dcrec/secp256k1/v4" fillcolor=mediumvioletred]
 	dcrutil [label="dcrutil/v4" fillcolor=burlywood]
+	mixing [label="mixing" fillcolor=lawngreen]
 	database [label="database/v3" fillcolor=dodgerblue]
 	hdkeychain [label="hdkeychain/v3" fillcolor=deepskyblue]
 	txscript [label="txscript/v4" fillcolor=mediumspringgreen]
@@ -34,7 +35,7 @@ digraph {
 
 	{rank=same blake256 chainhash}
 	{rank=same dcrec secp256k1}
-	{rank=same certgen bech32 apbf uint256}
+	{rank=same certgen bech32 apbf uint256 mixing}
 	{rank=same addrmgr connmgr hdkeychain peer blockchain standalone rpcclient}
 
 	certgen
@@ -57,6 +58,7 @@ digraph {
 	txscript -> peer [dir=back color=mediumspringgreen]
 	dcrutil -> blockchain [dir=back color=burlywood]
 	dcrutil -> database [dir=back color=burlywood]
+	dcrutil -> mixing [dir=back color=burlywood]
 	database -> stake [dir=back color=dodgerblue]
 	stake -> gcs [dir=back color=bisque]
 	gcs -> rpcclient [dir=back color=gold]
@@ -67,6 +69,7 @@ digraph {
 
 	rand -> dcrd [dir=back color=lavender]
 	blockchain -> dcrd [dir=back color=orchid]
+	mixing -> dcrd [dir=back color=lawngreen]
 	standalone -> dcrd [dir=back color=firebrick]
 	certgen -> dcrd [dir=back color=darkkhaki]
 	bech32 -> dcrd [dir=back color=antiquewhite3]