Add a pointer to the engineering design discussion forum.

Summary:
Add a pointer to the engineering design discussion forum.

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Task ID: #

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Author: dhruba

README

@@ -79,4 +79,4 @@ include/rocksdb/statistics.h
 include/rocksdb/transaction_log.h
     An API to retrieve transaction logs from a database.
 
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+Design discussions are conducted in https://www.facebook.com/groups/rocksdb.dev/