Add initial FAQ file

FAQ.md

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+FAQ - Frequently Asked Questions about repmgr
+=============================================
+
+
+General
+-------
+
+- What's the difference between the repmgr versions?
+
+  repmgr v3 builds on the improved replication facilities added
+  in PostgreSQL 9.3, as well as improved automated failover support
+  via `repmgrd`, and is not compatible with PostgreSQL 9.2 and earlier.
+
+  repmgr v2 supports PostgreSQL 9.0 onwards. While it is compatible
+  with  PostgreSQL 9.3 and later, we recommend repmgr v3.
+
+- What's the advantage of using replication slots?
+
+  Replication slots, introduced in PostgreSQL 9.4, ensure that the
+  master server will retain WAL files until they have been consumed
+  by all standby servers. This makes WAL file management much easier,
+  and if used `repmgr` will no longer insist on a fixed number (default: 5000)
+  of WAL files being preserved.
+
+  (However this does mean that if a standby is no longer connected to the
+  master, the master will retain WAL files indefinitely).
+
+`repmgr`
+--------
+
+- When should I use the --rsync-only option?
+
+  By default, `repmgr` uses `pg_basebackup` to clone a standby from
+  a master. However, `pg_basebackup` copies the entire data directory, which
+  can take some time depending on installation size. If you have an
+  existing but "stale" standby, `repmgr` can use `rsync` instead,
+  which means only changed or added files need to be copied.
+
+- Can I register an existing master/standby?
+
+  Yes, this is no problem.
+
+- Is there an easy way to check my master server is correctly configured
+  for use with `repmgr`?
+
+  Yes - execute `repmgr` with the `--check-upstream-config` option, and it
+  will let you know which items in `postgresql.conf` need to be modified.
+
+`repmgrd`
+---------
+
+- Do I need a witness server?
+
+  Not necessarily. However if you have an uneven number of nodes spread
+  over more than one network segment, a witness server will enable
+  better handling of a 'split brain' situation by providing a "casting
+  vote" on the preferred network segment.