Add FAQ to documentation

For the moment only copies the FAQ entries for installation from the wiki and adds one on running multiple extracts.

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+Frequently Asked Questions
+==========================
+
+Running Your Own Instance
+-------------------------
+
+### Can I import only a few countries and also keep them up to date?
+
+You should use the extracts and updates from http://download.geofabrik.de.
+For the intial import, download the countries you need and merge them.
+See [OSM Help](https://help.openstreetmap.org/questions/48843/merging-two-or-more-geographical-areas-to-import-two-or-more-osm-files-in-nominatim)
+for examples how to do that. Use the resulting single osm file when
+running `setup.php`.
+
+For updates you need to download the change files for each country
+once per day and apply them **separately** using
+
+    ./utils/update.php --import-diff <filename> --index
+    
+See [this issue](https://github.com/openstreetmap/Nominatim/issues/60#issuecomment-18679446)
+for a script that runs the updates using osmosis.
+
+### My website shows: `XML Parsing Error: XML or text declaration not at start of entity Location</code>.`
+
+Make sure there are no spaces at the beginning of your `settings/local.php` file.
+
+
+Installation
+------------
+
+### I accidentally killed the import process after it has been running for many hours. Can it be resumed?
+
+It is possible if the import already got to the indexing stage.
+Check the last line of output that was logged before the process
+was killed. If it looks like this:
+
+   Done 844 in 13 @ 64.923080 per second - Rank 26 ETA (seconds): 7.886255
+
+then you can resume with the following command:
+
+   ./utils/setup.php --index --create-search-indices --create-country-names
+
+If the reported rank is 26 or higher, you can also safely add `-index-noanalyse`.
+
+
+### When running the setup.php script I get a warning:
+    `PHP Warning:  file_get_contents(): open_basedir restriction in effect.`
+
+You need to adjust the [open_basedir](http://www.php.net/manual/en/ini.core.php#ini.open-basedir) setting
+in your PHP configuration (php.ini file). By default this setting may look like this:
+
+    open_basedir = /srv/http/:/home/:/tmp/:/usr/share/pear/
+
+Either add reported directories to the list or disable this setting temporarily by 
+dding ";" at the beginning of the line. Don't forget to enable this setting again
+once you are done with the PHP command line operations.
+
+
+### The Apache log contains lots of PHP warnings like this:
+    `PHP Warning:  date_default_timezone_set() function.`
+
+You should set the default time zone as instructed in the warning in
+your `php.ini` file. Find the entry about timezone and set it to
+something like this:
+  
+    ; Defines the default timezone used by the date functions
+    ; http://php.net/date.timezone
+    date.timezone = 'America/Denver'
+
+Or
+
+   echo "date.timezone = 'America/Denver'" > /etc/php.d/timezone.ini
+
+
+### When running the import I get a version mismatch:
+    `COPY_END for place failed: ERROR: incompatible library "/opt/Nominatim/module/nominatim.so": version mismatch`
+
+pg_config seems to use bad includes sometimes when multiple versions
+of PostgreSQL are available in the system. Make sure you remove the
+server development libraries (`postgresql-server-dev-9.1` on Ubuntu)
+and recompile (`cmake .. && make`).
+
+
+### I see the error: `function transliteration(text) does not exist`
+
+Reinstall the nominatim functions with `setup.php --create--functions`
+and check for any errors, e.g. a missing `nominatim.so` file.
+
+
+### The website shows: `Could not get word tokens`
+
+The server cannot access your database. Add `&debug=1` to your URL
+to get the full error message.
+
+
+### On CentOS the website shows `could not connect to server: No such file or directory`
+
+On CentOS v7 the PostgreSQL server is started with `systemd`.
+Check if `/usr/lib/systemd/system/httpd.service` contains a line `PrivateTmp=true`.
+If so then Apache cannot see the `/tmp/.s.PGSQL.5432` file. It's a good security feature,
+so use the [[#PostgreSQL_UNIX_Socket_Location_on_CentOS|preferred solution]] 
+
+However, you can solve this the quick and dirty way by commenting out that line and then run
+
+    sudo systemctl daemon-reload
+    sudo systemctl restart httpd
+
+
+### Setup.php fails with the message: `DB Error: extension not found`
+
+Make sure you have the Postgres extensions hstore and postgis installed.
+See the installation instruction for a full list of required packages.
+
+### When running the setup.php script I get a error:
+    `Cannot redeclare getDB() (previously declared in /your/path/Nominatim/lib/db.php:4)`
+
+The message is a bit misleading as PHP needs to load the file `DB.php` and
+instead re-loads Nominatim's `db.php`. To solve this make sure you
+have the [http://pear.php.net/package/DB/ Pear module 'DB'] installed.
+
+    sudo pear install DB
+
+### I forgot to delete the flatnodes file before starting an import
+
+That's fine. For each import the flatnodes file get overwritten.
+See https://help.openstreetmap.org/questions/52419/nominatim-flatnode-storage
+for more information.