Added HACKING.postgres

HACKING.postgres

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+PostgreSQL depends on some environment variables and directories which
+are easiest to have via a .bashrc like this:
+
+export PG_PREFIX=/home/shackle/tip
+export PGDATA=$PG_PREFIX/data
+export PATH=$PG_PREFIX/bin:$PATH
+export PGPORT=2225
+export PGUSER=shackle
+export PGDATABASE=postgres
+
+When you want to work on CVS TIP (aka git master) of PostgreSQL, you'd
+source this file first.  If you want to change to another
+PostgreSQL, do:
+
+    make distclean
+    . /path/to/new/.bashrc.whatever
+
+To avoid confusion, you also might want to un-symlink plparrot from
+beneath the other source tree and symlink it under contrib/ in the
+source tree you want to use.