Add guidelines for contributing

[vickumar1981]

Need to add a CONTRIBUTING.md file that outlines:

• How and where to install scala and sbt (simple build tool, aka maven, but for scala and also much better) Links: https://www.scala-sbt.org/1.0/docs/Setup.html, https://www.scala-lang.org/download/2.12.7.html

Alternatively, could include the sbt-launch.jar file in the project, and the only dependency would be having Java 8. (https://github.com/sbt/launcher)

• Need to include instructions about forking/branching from latest snapshot branch and opening PR's against the snapshot branch (currently, 1.0.8-SNAPSHOT is the branch)

• Instructions on how to run unit tests (sbt test runs unit tests) and the ./test.sh command will run the linter (scalastyle), unit tests, generate a coverage report, and check that code coverage is adequate (100%).

• Instructions on how to generate and check documentation (sbt doc will create the documentation in your local /target folder of the project)

• Code style and conventions - Could use https://github.com/excellalabs/intro-to-functional-scala/blob/master/scala-style-guide.md or https://github.com/databricks/scala-style-guide

• PR etiquette and general guidelines

[vickumar1981]

sbt-launcher.jar is now included in v1.0.8 from: https://github.com/vickumar1981/stringdistance/pull/21

[vickumar1981]

Addressed by: https://github.com/vickumar1981/stringdistance/pull/27

Will be in 1.0.8