Add info about plugin's update

src/plugins/update.md

@@ -0,0 +1,48 @@
+---
+title: Update
+show_in_menu: contents
+nav_sort: 10
+---
+
+# Plugin update
+
+Update procedure can vary from plugin to plugin but the common steps are the
+same and listed below. Also it's recommended to check README file shipped
+with a plugin before update it.
+
+
+## The process
+
+### Get the plugin
+
+First of all you need to download new version of plugin's sources. In the most
+cases plugin's author provides prepared `.zip` or `.tar.gz` archive with the
+plugin sources.
+
+All officially approved plugins are listed at https://mibew.org/plugins page.
+
+
+### Update sources
+
+When you've got the plugin's archive you should extract files from it and
+replace old plugin's files with new ones.
+
+For example, files of "**FooCorp:Connector**" plugin should be situated in
+`<mibew base path>/plugins/FooCorp/Mibew/Plugin/Connector/` directory.
+
+**Warning**: Letters case in the path is important!
+
+It's highly recommended to backup files before replace them.
+
+
+### Update database
+
+After plugin's files are updated the database should be updated too. To do so
+navigate to `<mibew base URL>/operator/plugin` page and click on "_update_" link
+in plugin's row.
+
+It's highly recommended to backup database before the update.
+
+If the plugin was updated correctly the value in "_state_" column will be
+changed to "_working_". If it does not check apache's error log for details
+about the problem.