docs: make customization chapter a separate section

docs/customize/Special-Phrases.md

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+# Special phrases
+
+## Importing OSM user-maintained special phrases
+
+As described in the [Importation chapter](Import.md), it is possible to
+import special phrases from the wiki with the following command:
+
+```sh
+nominatim special-phrases --import-from-wiki
+```
+
+## Importing custom special phrases
+
+But, it is also possible to import some phrases from a csv file. 
+To do so, you have access to the following command:
+
+```sh
+nominatim special-phrases --import-from-csv <csv file>
+```
+
+Note that the two previous import commands will update the phrases from your database.
+This means that if you import some phrases from a csv file, only the phrases
+present in the csv file will be kept into the database. All other phrases will
+be removed.
+
+If you want to only add new phrases and not update the other ones you can add
+the argument `--no-replace` to the import command. For example:
+
+```sh
+nominatim special-phrases --import-from-csv <csv file> --no-replace
+```
+
+This will add the phrases present in the csv file into the database without
+removing the other ones.