Add threat-modeling section to C# MaD docs

Author: egregius313

docs/codeql/codeql-language-guides/customizing-library-models-for-csharp.rst

@@ -340,3 +340,19 @@ The first four values identify the callable (in this case the getter of the ``No
 - The fourth value ``()`` is the method input type signature.
 - The fifth value ``summary`` is the kind of the neutral.
 - The sixth value ``manual`` is the provenance of the neutral.
+
+.. _threat-models-csharp:
+
+Threat models
+-------------
+
+.. include:: ../reusables/beta-note-threat-models.rst
+
+A threat model is a named class of dataflow sources that can be enabled or disabled independently. Threat models allow you to control the set of dataflow sources that you want to consider unsafe. For example, one codebase may only consider remote HTTP requests to be tainted, whereas another may also consider data from local files to be unsafe. You can use threat models to ensure that the relevant taint sources are used in a CodeQL analysis.
+
+The ``kind`` property of the ``sourceModel`` determines which threat model a source is associated with. There are two main categories:
+
+- ``remote`` which represents requests and responses from the network. 
+- ``local`` which represents data from local files (``file``), command-line arguments (``commandargs``), database reads (``database``), and environment variables(``environment``).
+ 
+When running a CodeQL analysis, the ``remote`` threat model is included by default. You can optionally include other threat models as appropriate when using the CodeQL CLI and in GitHub code scanning. For more information, see `Analyzing your code with CodeQL queries <https://docs.github.com/code-security/codeql-cli/getting-started-with-the-codeql-cli/analyzing-your-code-with-codeql-queries#including-model-packs-to-add-potential-sources-of-tainted-data>`__ and `Customizing your advanced setup for code scanning <https://docs.github.com/code-security/code-scanning/creating-an-advanced-setup-for-code-scanning/customizing-your-advanced-setup-for-code-scanning#extending-codeql-coverage-with-threat-models>`__.