Add .gitignore for Apache Freemarker

[akshaysrivastava113]

Reasons for making this change: Repo had not .gitignore to support Apache Freemarker code.

Links to documentation supporting these rule changes:

https://freemarker.apache.org/sourcecode.html https://gitbox.apache.org/repos/asf?p=freemarker-site.git https://github.com/apache/freemarker-site/blob/master/.gitignore