Add mention of history archive path to readme (#874)

Author: leighmcculloch

README.md

@@ -57,6 +57,7 @@ HTTP APIs and Tools are available at the following port and paths:
 - RPC: `http://localhost:8000/rpc`
 - Lab: `http://localhost:8000/lab`
 - Friendbot: `http://localhost:8000/friendbot`
+- History Archive: `http://localhost:8000/archive` (available with `--local` only)
 
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