Add Update documentation page

doc/Update.md

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+Updating Friendica
+===============
+
+* [Home](help)
+
+## Using a Friendica archive
+
+If you installed Friendica in the ``path/to/friendica`` folder:
+1. Unpack the new Friendica archive in ``path/to/friendica_new``.
+2. Copy ``.htconfig.php``, ``photo/`` and ``proxy/`` from ``path/to/friendica`` to ``path/to/friendica_new``.
+3. Rename the ``path/to/friendica`` folder to ``path/to/friendica_old``.
+4. Rename the ``path/to/friendica_new`` folder to ``path/to/friendica``.
+5. Check your site. Note: it may go into maintenance mode to update the database schema.
+6. If everything works, just delete the ``path/to/friendica_old`` folder.
+
+To update Addons from an archive, simply delete the ``path/to/friendica/addon`` and replace it with the provided archive.
+
+## Using Git
+
+You can get the latest changes at any time with
+
+    cd path/to/friendica
+    git pull
+    util/composer.phar install
+
+The addon tree has to be updated separately like so:
+
+    cd path/to/friendica/addon
+    git pull
+
+For both repositories:
+The default branch to use is the ``master`` branch, which is the stable version of Friendica.
+It is updated about four times a year on a fixed schedule.
+
+If you want to use and test bleeding edge code please checkout the ``develop`` branch.
+The new features and fixes will be merged from ``develop`` into ``master`` after a release candidate period before each release.
+
+Warning: The ``develop`` branch is unstable, and breaks on average once a month for at most 24 hours until a patch is submitted and merged.
+Be sure to pull frequently if you choose the ``develop`` branch.
+
+### Considerations before upgrading Friendica
+
+#### MySQL >= 5.7.4
+
+Starting from MySQL version 5.7.4, the IGNORE keyword in ALTER TABLE statements is ignored.
+This prevents automatic table deduplication if a UNIQUE index is added to a Friendica table's structure.
+If a DB update fails for you while creating a UNIQUE index, make sure to manually deduplicate the table before trying the update again.
+
+#### Manual deduplication
+
+There are two main ways of doing it, either by manually removing the duplicates or by recreating the table.
+Manually removing the duplicates is usually faster if they're not too numerous.
+To manually remove the duplicates, you need to know the UNIQUE index columns available in `database.sql`.
+
+```SQL
+SELECT GROUP_CONCAT(id), <index columns>, count(*) as count FROM users
+GROUP BY <index columns> HAVING count >= 2;
+
+/* delete or merge duplicate from above query */;
+```
+
+If there are too many rows to handle manually, you can create a new table with the same structure as the table with duplicates and insert the existing content with INSERT IGNORE.
+To recreate the table you need to know the table structure available in `database.sql`.
+
+```SQL
+CREATE TABLE <table_name>_new <rest of the CREATE TABLE>;
+INSERT IGNORE INTO <table_name>_new SELECT * FROM <table_name>;
+DROP TABLE <table_name>;
+RENAME TABLE <table_name>_new TO <table_name>;
+```
+
+This method is slower overall, but it is better suited for large numbers of duplicates.