Add the solana-wallet documentation (#1744)

* Add the solana-wallet documentation

There doesn't seem to be a way to publish bin docs to crates.io.
Until there is, we can include CLI documentation is the appendix
of the markdown book.

* A command to generate all the usage docs

Usage:

$ scripts/wallet-help.sh >> src/wallet.md

scripts/wallet-help.sh

@@ -0,0 +1,20 @@
+#!/bin/bash -e
+
+cd "$(dirname "$0")"/..
+
+cargo build
+export PATH=$PWD/target/debug:$PATH
+
+echo "\`\`\`manpage"
+solana-wallet --help
+echo "\`\`\`"
+echo ""
+
+commands=(address airdrop balance cancel confirm deploy get-transaction-count pay send-signature send-timestamp)
+
+for x in "${commands[@]}"; do
+    echo "\`\`\`manpage"
+    solana-wallet "${x}" --help
+    echo "\`\`\`"
+    echo ""
+done