Code block cleanup on README.md

1. Changed the indentation from a mix of tabs and spaces to four spaces for more predictable editing.
2. Use [GitHub-style Markdown code blocks](https://help.github.com/articles/github-flavored-markdown#syntax-highlighting)
3. Gave `username_of_recipient` a data type of String (there was none before)
4. Refactored the variable named `username_of_recipient` to `recipientUsername` to conform with the Java naming conventions.
5. Added import statements to allow the code to be able to be copied and pasted and compile correctly