Update README.md

Author: swhitley

README.md

@@ -15,9 +15,10 @@ Here are a few ideas. (Care to add to this list in the discussions?)
 ## Installation
 
 1. Download the [latest release](https://github.com/swhitley/CLARiNET/releases/latest) for your operating system.
-2. Unzip the executable files into a new directory.
-3. Run `clarinet --help` to view the available options.
-4. Download [Test.clar](https://github.com/swhitley/CLARiNET/blob/main/Test.clar) to try it out.
+3. Unzip the executable files into a new directory.
+4. Run `clarinet --help` to view the available options.
+5. You may need to install the [dotnet core runtime](https://dotnet.microsoft.com/download/dotnet/5.0/runtime).
+6. Optional: Download [Test.clar](https://github.com/swhitley/CLARiNET/blob/main/Test.clar) to try it out.
 
 ## Loading a CLAR file to Workday