Document how to save disk space on our bitcoind.

wiki/src/contribute/working_together/roles/sysadmins.mdwn

@@ -192,6 +192,10 @@ Below, importance level is evaluated based on:
 * configuration: `bitcoind` class in [[!tails_gitweb_repo puppet-bitcoind]]
 * Vcs-Git: <https://git-tails.immerda.ch/bitcoin/> and <https://git-tails.immerda.ch/bitcoin_libunivalue/>
 * importance: medium
+* To save disk space: as the `bitcoin@bitcoin.lizard` user, run
+  `bitcoin-cli getblockcount` to get the ID of the last block,
+  then run `bitcoin-cli pruneblockchain XYZ`, with `XYZ` being
+  a Unix timestamp that's at least 5 months in the past.
 
 ## BitTorrent