add chain var three documentation

audit/var-3.md

@@ -0,0 +1,32 @@
+# Chain Variable Transaction 3
+
+## Changes
+
+This transaction changes the `monthly_reward` variable to 5000000000000 from 50000000000.
+
+## Rationale
+
+In the genesis block, there an error was made.  The initial `monthly_reward` was set to 50000000000.  Because of this, tokens are mined at a rate that is one hundred times too slow.
+
+## Version threshold
+
+None
+
+## Transaction
+
+```
+{blockchain_txn_vars_v1_pb,
+    [{blockchain_var_v1_pb,"monthly_reward","int",
+         <<"5000000000000">>}],
+    0,
+    <<48,68,2,32,24,78,173,139,96,93,22,38,189,104,85,19,213,
+      224,74,116,89,1,14,31,176,148,62,11,69,17,14,3,154,207,
+      70,125,2,32,84,184,71,55,100,56,155,254,85,236,102,2,
+      251,154,0,190,171,207,39,51,5,74,51,193,181,133,79,85,
+      43,14,40,127>>,
+    <<>>,<<>>,[],[],3}
+```
+
+## Acceptance block
+
+6116