build: Support manually generating C# bindings via Github action and add a README (#423)

bindings/csharp/README.md

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+# Regorus CSharp
+
+**Regorus** is
+
+  - *Rego*-*Rus(t)*  - A fast, light-weight [Rego](https://www.openpolicyagent.org/docs/latest/policy-language/)
+   interpreter written in Rust.
+  - *Rigorous* - A rigorous enforcer of well-defined Rego semantics.
+
+See main [Regorus page](https://github.com/microsoft/regorus) for more details about the project.
+
+# Building
+
+## Github Actions
+
+The simplest way to build a Nuget for Regorus' C# bindings is to use Github Actions. The action to do so is named `bindings/csharp` and is defined in `.github/workflows/test-csharp.yml`.
+
+There are two ways to trigger a Nuget build.
+1. Runs are triggered automatically whenever a push or pull request is made to the `main` branch.
+2. A run can be triggered manually by navigating to the action in the Github UI and clicking `Run workflow`. This option allows you to generate a Nuget for any branch, which is useful when testing the integration of in-progress changes to Regorus with other projects. Nuget files that are generated via this flow will have a `manualtrigger` suffix appended to their version number, making it easy to distinguish them from Nugets generated using the `main` branch.
+![Image displaying the run workflow button](docs/images/readme/manuallytriggeringrun.png)
+
+Once the workflow run completes, the generated Nuget can be downloaded by following these steps:
+1. Open the run.
+2. Click on `Build Regorus nuget` on the left.
+3. Expand the `Upload Regorus nuget` step.
+4. Click the `Artifact download URL` link at the bottom.
+5. Save and extract the downloaded zip file to find the `.nupkg` file.
+![Image displaying the download URL link](docs/images/readme/downloadnuget.png)
+
+## Local
+
+TODO