doc: Add historical release notes

Add all known release notes for versions since 0.3.12,
thanks to Matt Corallo for collecting them on the wiki.

doc/release-notes/release-notes-0.3.18.md

@@ -0,0 +1,11 @@
+Changes:
+* Fixed a wallet.dat compatibility problem if you downgraded from 0.3.17 and then upgraded again
+* IsStandard() check to only include known transaction types in blocks
+* Jgarzik's optimisation to speed up the initial block download a little
+
+The main addition in this release is the Accounts-Based JSON-RPC commands that Gavin's been working on (more details at http://www.bitcoin.org/smf/index.php?topic=1886.0).  
+* getaccountaddress
+* sendfrom
+* move
+* getbalance
+* listtransactions