docs: Add some full-text search documentation.

docs/full-text-search.md

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+# Full-text search
+
+Zulip supports full-text search, which can be combined arbitrarily
+with Zulip's full suite of narrowing operators.  By default, it only
+supports English text, but there is an experimental
+[PGroonga](http://pgroonga.github.io/) integration that provides
+full-text search for all languages.
+
+The user interface and feature set for Zulip's full-text search is
+documented in the "Search help" documentation section in the Zulip
+app's gear menu.
+
+## The default full-text search implementation
+
+Zulip's uses [PostgreSQL's built-in full-text search
+feature](http://www.postgresql.org/docs/current/static/textsearch.html),
+with a custom set of English stop words to improve the quality of the
+search results.
+
+We use a small extension,
+[tsearch_extras](https://github.com/zulip/tsearch_extras), for
+highlighting of the matching words.  There is [some discussion of
+removing this extension, at least as an
+option](https://github.com/zulip/zulip/issues/467), so that Zulip can
+be used with database-as-a-service platforms.
+
+In order to optimize the performance of delivering messages, the
+full-text search index is updated for newly sent messages in the
+background, after the message has been delivered.  This background
+updating is done by
+`puppet/zulip/files/postgresql/process_fts_updates`, which is usually
+deployed on the database server, but could be deployed on an
+application server instead.
+
+## An optional full-text search implementation
+
+See [the option PGroonga pull
+request](https://github.com/zulip/zulip/pull/700) for details on the
+status of the PGroonga integration.