Add info about plugin's installation

src/plugins/installation.md

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+---
+title: Installation
+show_in_menu: contents
+nav_sort: 0
+---
+
+# Plugin installation
+
+Installation 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 install it.
+
+
+## The process
+
+### Get the plugin
+
+First of all you need to download the plugin 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.
+
+
+### Upload the plugin
+
+When you've got the plugin's archive you should extract files from it and upload
+them to the Mibew's server. The files of the plugin should be in
+`<mibew base path>/plugins/<Vendor name>/Mibew/Plugin/<Plugin name>/` directory.
+
+For example, the main file of "**FooCorp:Connector**" plugin should have
+`<mibew base path>/plugins/FooCorp/Mibew/Plugin/Connector/Plugin.php` path.
+
+**Warning**: Letters case in the path is important!
+
+
+### Configure the plugin
+
+A plugin may require additional configuration. Check plugin's description or
+README file that shipped with plugin's archive to know if the plugin needs to be
+configured.
+
+All configuration value are set in `configs/config.yml` file. If you are not
+familiar with YAML syntax take a look at https://en.wikipedia.org/wiki/YAML or
+any other description that can be find on the Internet.
+
+Plugins configurations are stored in `plugins` associative array, which keys are
+fully qualified plugins' names and the values are plugins' options. If there is
+no configurable plugins installed the `plugins` array is looks like:
+
+```yaml
+plugins: []
+```
+
+Lets assume you are trying to install "**FooCorp:Connector**" plugin that requires
+`timeout` and `reconnects` options. The `plugins` array from `configs/config.yml`
+file becomes:
+
+```yaml
+plugins:
+    "FooCorp:Connector":
+        timeout: 200
+        reconnects: 3
+```
+
+Also there are special configuration option `weight` that can be applied to
+any plugin and controls order in which plugins are loaded. A plugin with lower
+weight will be loaded and executed earlier than the one with higher weight. This
+option can be omitted and equals to `0` by default.
+
+
+### Install the plugin
+
+After the plugin's files are in place and the plugin is configured it should be
+installed using Mibew web interface.
+
+To do so you should navigate to `<mibew base URL>/operator/plugin` page and
+click on "_enable_" link in plugin's row.
+
+![Enable plugin link](../assets/images/enable-plugin.png)
+
+If the plugin was installed and enabled 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.