PS: Add readme with slight modifications from the internal repo.

Author: MathiasVP

powershell/README.md

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+# Powershell Extractor
+
+## Directories:
+- `extractor`
+  - Powershell extractor source code
+- `ql`
+  - QL libraries and queries for Powershell (to be written)
+- `tools`
+  - Directory containing files that must be copied to powershell/tools in the directory containing the CodeQL CLI. This will be done automatically by `build.ps1` (see below).
+
+## How to build the Powershell:
+- Run `build.ps1 path-to-codeql-cli-folder` where `path-to-codeql-cli-folder` is the path to the folder containing the CodeQL CLI (i.e., `codeql.exe`).