Update contrib/README

Remove lots of outdated information that is duplicated by the
better-maintained SGML documentation.  In particular, remove the
outdated listing of contrib modules.  Update the installation
instructions to mention CREATE EXTENSION, but don't go into too much
detail.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/48ea558313fae4409d0e76facde041674678cba2

Modified Files
--------------
contrib/README |  219 +++++---------------------------------------------------
1 files changed, 18 insertions(+), 201 deletions(-)

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