[DOC][I] Adjust installation details

Adjusts the installation details for Saros/I to point out that the user
needs to add the alpha release channel to their list or plugin
repositories to find our plugin in the market place.

Author: tobous

README.md

@@ -16,7 +16,7 @@ Is an Open Source plugin for connecting multiple IDEs for distributed collaborat
 ### IntelliJ alpha release
 Please read the [details about restrictions](https://www.saros-project.org/releases/saros-i_0.1.0.html) first, because Saros for IntelliJ is still a first alpha release!
 
-- Search for "Saros" in the IntelliJ Marketplace
+- Search for "Saros" in the IntelliJ Marketplace (this requires the `alpha` release channel; more information on this is given in the [release notes](https://www.saros-project.org/releases/saros-i_0.1.0.html#installation))
 - Or Download the plugin zip from our [release page](https://github.com/saros-project/saros/releases)
 
 ## Where to get help

docs/releases/saros-i_0.1.0.md

@@ -28,6 +28,7 @@ Saros/I can be installed from the JetBrains plugin repository or from disk.
 - Choose "Settings..." > "Plugins".
 - Choose a source to install from:
   - *From the plugin repository:* Select the tab "Marketplace"
+    - If you have not already done so, add the `alpha` release channel to your list of plugin repositories. A guide on how to do this is given [here](https://www.jetbrains.org/intellij/sdk/docs/plugin_repository/custom_channels.html#configuring-a-custom-channel-in-intellij-platform-based-ides).
     - Search for "Saros" in the search bar and select the entry from the list.
     - Click the "Install" button.
     - Close the settings menu.