add usage instructions to README

This project is a copy of Erlang's JInterface library to link Erlang and Java programs,

packaged for Maven in order to use it in other projects.

Available versions:

## Usage

Add the following to your `pom.xml` file:

```xml

<project>

  <!-- ... -->

  <dependencies>

    <dependency>

      <groupId>program-slicing</groupId>

      <artifactId>erlang-jinterface</artifactId>

      <version>VERSION</version>

    </dependency>

  </dependencies>

  <repositories>

    <repository>

      <id>mist-gitlab-jinterface</id>

      <url>https://mist.dsic.upv.es/git/api/v4/projects/45/packages/maven</url>

    </repository>

  </repositories>

</project>

```

Where `VERSION` is a value from the first column on the following table,

matching the version of Erlang you have installed.

| JInterface | Erlang versions supported |

|------------|---------------------------|