Update Src/README.md

Author: Lamparter

Src/README.md

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+# `src`
+
+- `IronPython`: Core compiler and runtime (IronPython.dll)
+
+IronPython.Tests: Targetted compiler and runtime unit tests hosted in a C# test harness.
+
+`Scripts`: Infrastructure scripts and helpers.
+
+`StdLib`: Python standard library imports.
+
+`Tests`: Test suites.
+
+Please see [the wiki](https://github.com/IronLanguages/ironpython3/wiki) for information on:
+
+- Setting up a development environment with easy access to utility scripts
+- Building
+- Running test