message: added known issue for Unity
diff: ## Known Issues in 1.23.1
  +* For Mono-based development (e.g. Unity) that requires full .NET framework, you need to set `"omnisharp.useGlobalMono": "always"`. This is needed until Mono [upgrades their bundled MSBuild version](https://github.com/mono/mono/issues/20250)
  * Known limitations with the preview Razor (cshtml) language service to be addressed in a future release:
  * Only ASP.NET Core projects are supported (no support for ASP.NET projects)
  * Limited support for formatting
  * Renaming symbol fails within a file that had recently been renamed without saving changes.
  * As a workaround, make an edit within the file before using Rename Symbol.
  -## 1.23.1 (Not yet released)
  +## 1.23.1 (August 19, 2020)
  * Register FixAll commands for disposal ([#3984](https://github.com/OmniSharp/omnisharp-vscode/issues/3984), PR: [#3985](https://github.com/OmniSharp/omnisharp-vscode/pull/3985))
  * Include version matched target files with minimal MSBuild (PR: [omnisharp-roslyn#1895](https://github.com/OmniSharp/omnisharp-roslyn/pull/1895))
  * Fix lack of trailing italics in quickinfo (PR: [omnisharp-roslyn#1894](https://github.com/OmniSharp/omnisharp-roslyn/pull/1894))
  