Add src/veris/README.md

src/veris/README.md

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+# Tools for VERIS Mappings to MITRE ATT&CK®
+
+This directory contains a Python package of tools for working with the VERIS mappings to
+ATT&CK.
+
+## Set up
+
+You will need the following prerequisites to run the tools:
+
+1. [Python (≥3.8)](https://www.python.org/downloads/)
+2. [Python Poetry](https://python-poetry.org/docs/#installation)
+
+Once you have the repository cloned, run the following one-time command to initialize a
+virtual environment and install dependenies:
+
+```
+poetry install
+```
+
+Once the dependencies are installed, you will need to source the environment in each
+terminal window prior to using the Python tools.
+
+```
+poetry shell
+```
+
+## Tools
+
+The tools are organized into the following subdirectories. Click the link to view
+detailed instructions for working with those tools.
+
+| Directory                     | Description                                                                                    |
+| ----------------------------- | ---------------------------------------------------------------------------------------------- |
+| [`parse/`](./parse)           | Tools for parsing VERIS data and mappings spreadsheets.                                        |
+| [`util/`](./util)             | Utilities for generating content from mappings data, such as Navigator layers, CSV files, etc. |
+| [`veris_diff/`](./veris_diff) | Tools for comparing versions of the VERIS schema and identifying changes.                      |
+
+## Customization
+
+To create customized mappings, edit the input data in the [`inputs/`](../../inputs)
+directory and then use the tools above to regenerate the outputs.