Update install/update documentation

UPDATE.md

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+# Friendica Directory Update Instructions
+
+## 1. Update the source code
+
+If you installed Friendica Directory in `/path/to/friendica-directory`.
+
+### Using Git
+
+```
+cd /path/to/friendica-directory
+git pull
+composer install
+```
+
+### Using an archive
+
+1. Create a temporary folder to unpack the new archive.
+2. Copy your old `config/local.json` to the new folder.
+3. Swap the folder names.
+4. Remove the temporary folder.
+
+Sample Linux commands:
+```
+cd /path/to
+mkdir friendica-directory-new
+unzip friendica-<version>.zip friendica-directory-new
+cp friendica-directory/config/local.json friendica-directory-new/config
+mv friendica-directory friendica-directory-old
+mv friendica-directory-new friendica-directory
+rm -r friendica-directory-old
+```
+
+## 2. Update the database structure
+
+The database structure may have changed since the last update, fortunately a console command allows to run the migration scripts up to the latest version:
+
+```
+cd /path/to/friendica-directory
+bin/console dbupdate
+```
+
+You're all set!