Added README.

Author: haydenflinner

README.md

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+# Spreadsheet Mapping Tool
+Given a `.xlsx` file, gives you a graph of the cell formula dependencies. Useful for visualizing the inner workings of large spreadsheets.
+
+![Example rendered graph](/example_output.png?raw=true "Example Graph")
+
+## Getting Started
+
+These instructions will get you a copy of the project up and running on your local machine for development and usage purposes.
+
+### Prerequisites
+Java
+
+Gradle (to build, otherwise grab a `.jar` from Releases
+
+Graphvis (sudo apt install graphviz)
+
+
+### Building (Skip to Using just interested in basic use)
+
+You can import the project as an IntelliJ project, or if you prefer the command line, just
+
+```
+gradle jar
+```
+
+### Using
+Run the tool from the command line and pipe the output into graphvis:
+
+```
+# java -jar excel-mapper.jar test.xlsx | dot -Tpng -o example_output.png
+```
+
+
+## Built With
+
+* [Apache POI](https://poi.apache.org/) - For all of the gnarly formula parsing
+* [Gradle](https://maven.apache.org/) - Dependency Management
+
+## Contributing
+
+Send in a pull request with whatever you'd like. Additions necessary:
+* Test with non-Apache-POI-supported functions in the spreadsheet, like custom VisualBasic functions.
+  * This should work fine since we're not actually evaluating any functions, only cell formula contents, but is untested.
+
+This project is licensed under the MIT License.
+