Updating install instructions (#806)

Updating install instructions to consider the slight differences with the pkg manager between 2020 and 2021 engine versions.
2020.3 does not have an option for add by name. Details added to help users install when using Unity 2020.3.

Co-authored-by: Larah Armstrong <[email protected]>

Author: chrispope

docs/installation/installation.md

@@ -41,7 +41,8 @@ Use with caution:
 
 1. Open your **Unity Hub** and select the **Project** you are presently working on or create a **New Project**.
 1. From the menu bar, navigate to **Window** > **Package Manager**.
-1. Click the plus sign ![Add](/img/add.png) in the **Package Manager** status bar and select **Add package by name..**.
+1. When using 2021.3+ click the plus sign ![Add](/img/add.png) in the **Package Manager** status bar and select **Add package by name**. 
+1. If using 2020.3LTS there is no option to add by package name, instead click **Add package from git URL**.
 
  ![Package Installed](/img/install/addbyname.png)
 
@@ -62,4 +63,4 @@ See the following content to continue your journey using Netcode:
   * [Boss Room](../learn/bossroom/getting-started-boss-room.md)
   * [2D Spaceshooter Bitesize Sample](../learn/bitesize/bitesize-spaceshooter.md)
   * [Invaders Bitesize Sample](../learn/bitesize/bitesize-invaders.md)
-  * [Client-Driven Bitesize Sample](../learn/bitesize/bitesize-clientdriven.md)
+  * [Client-Driven Bitesize Sample](../learn/bitesize/bitesize-clientdriven.md)