Update readme to cover how to update the dotnet global tool

Author: sleeuwen

README.md

@@ -17,6 +17,11 @@ To install dotnet-json as a global tool, run the following command:
 dotnet tool install -g dotnet-json
 ```
 
+Or, if you already have `dotnet-json` installed and want to update it to the latest version, run the following command:
+```
+dotnet tool update -g dotnet-json
+```
+
 ## Usage
 
 When you installed dotnet-json as a .NET Core global tool, you can run it as either `dotnet json` or `dotnet-json`.

dotnet-json.Tests/dotnet-json.Tests.csproj

@@ -4,6 +4,7 @@
     <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>dotnet_json.Tests</RootNamespace>
     <IsPackable>false</IsPackable>
+    <IsPublishable>false</IsPublishable>
 
     <LangVersion>8.0</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>