chore: modify readme to include basic information (#129)

Author: JamesRTaylor

readme.md

@@ -1,2 +1,27 @@
-Substrait Java Code
+# Substrait Java
 
+Substrait Java is a project that makes it easier to build [Substrait](https://substrait.io/) plans through Java. The project has two main parts:
+1) **Core** is the module that supports building Substrait plans directly through Java. This is much easier than manipulating the Substrait protobuf directly. It has no direct support for going from SQL to Substrait (that's covered by the second part)
+2) **Isthmus** is the module that allows going from SQL to a Substrait plan. Both Java APIs and a top level script for conversion are present. Not all SQL is supported yet by this module, but a lot is. For example, all of the TPC-H queries and all but a few of the TPC-DS queries are translatable.
+
+## Building
+After you've cloned the project through git, Substrait Java is built with a tool called [Gradle](https://gradle.org/). To build, execute the following:
+```
+./gradlew build
+```
+
+To build the Isthmus executable that enables Substrait plans to be generated for a SQL statement, execute the following:
+```
+./gradlew nativeImage
+```
+
+## Getting Started
+A good way to get started is to experiment with building Substrait plans for your own SQL. To do that, you can use the isthmus executable as described [here](https://github.com/substrait-io/substrait-java/blob/main/isthmus/README.md).
+
+Another way to get an idea of what Substrait plans look like is to use our script that generates Substrait plans for all the TPC-H queries:
+```
+./isthmus/src/test/script/tpch_smoke.sh
+```
+
+## Getting Involved
+To learn more, head over [Substrait](https://substrait.io/), our parent project and join our [community](https://substrait.io/community/)