doc: add contrib/signet readme

contrib/signet/README.md

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+Contents
+========
+This directory contains tools related to Signet, both for running a Signet yourself and for using one.
+
+getcoins.py
+===========
+
+A script to call a faucet to get Signet coins.
+
+Syntax: `getcoins.py [-h|--help] [-c|--cmd=<bitcoin-cli path>] [-f|--faucet=<faucet URL>] [-a|--addr=<signet bech32 address>] [-p|--password=<faucet password>] [--] [<bitcoin-cli args>]`
+
+* `--cmd` lets you customize the bitcoin-cli path. By default it will look for it in the PATH
+* `--faucet` lets you specify which faucet to use; the faucet is assumed to be compatible with https://github.com/kallewoof/bitcoin-faucet
+* `--addr` lets you specify a Signet address; by default, the address must be a bech32 address. This and `--cmd` above complement each other (i.e. you do not need `bitcoin-cli` if you use `--addr`)
+* `--password` lets you specify a faucet password; this is handy if you are in a classroom and set up your own faucet for your students; (above faucet does not limit by IP when password is enabled)
+
+If using the default network, invoking the script with no arguments should be sufficient under normal
+circumstances, but if multiple people are behind the same IP address, the faucet will by default only
+accept one claim per day. See `--password` above.