Update Readme

Author: jools-r

README.md

@@ -15,14 +15,18 @@ example:
 -   Store associated details, for example the author(s) of a document,
     year of issue, duration of a music track ...
 
-## Installation / Deinstallation
+## Installation / Deinstallation / Upgrading
 
 ### Installation
 
 Paste the `.txt` installer code into the *Admin › Plugins* panel, or
 upload the plugin's `.php` file via the *Upload plugin* button, then
 install and enable the plugin.
 
+### Upgrading
+
+The plugin automatically migrates custom field data and the database structure from the earlier single custom field variant (v0.1) to the new format. No changes are needed to the public tags as the new default settings correspond to the old tag. Nevertheless, it is always advisable to make a database backup before upgrading.
+
 ### De-installation
 
 The plugin cleans up after itself: deinstalling (deleting) the plugin

README.textile

@@ -10,12 +10,16 @@ Use whenever extra information needs to be stored with a file. For example:
 * Store associated details, for example the author(s) of a document, year of issue, duration of a music track …
 
 
-h2. Installation / Deinstallation
+h2. Installation / Deinstallation / Upgrading
 
 h3. Installation
 
 Paste the @.txt@ installer code into the _Admin › Plugins_ panel, or upload the plugin's @.php@ file via the _Upload plugin_ button, then install and enable the plugin.
 
+h3. Upgrading
+
+The plugin automatically migrates custom field data and the database structure from the earlier single custom field variant (v0.1) to the new format. No changes are needed to the public tags as the new default settings correspond to the old tag. Nevertheless, it is always advisable to make a database backup before upgrading.
+
 h3. De-installation
 
 The plugin cleans up after itself: deinstalling (deleting) the plugin removes the extra columns from the database as well as custom field names and labels. To stop using the plugin but keep the custom field data in the database, just disable (deactivate) the plugin but don't delete it.