Add Racket.gitignore

[willprice]

Reasons for making this change:

Add Racket support (similar to Lisp/Scheme)

Links to documentation supporting these rule changes:

• DrRacket files (*.rkt~, *.rkt.back, #*.rkt#*#)
• raco bytecode (compiled/, *.zo)
• raco dependency tracking files (compiled/, *.dep)

Commit Message
Dr Racket the official Racket IDE produces quite a large number of
temporary files (see: https://docs.racket-lang.org/drracket/drracket-files.html )

• Backup files are the src filename + ~ on UNIX and .bak on Windows
• DrRacket autosave files are src filename prepended with #, suffixed
  with #, a number and then another #.
• raco, the Racket CLI build/package/test tool produces a compiled
  directory to store bytecode. It also produces *.dep and *.zo
  files.

Noteworthy
Although similar to Scheme, Racket has it's own file suffixes (.rkt) and is a much larger and versatile language with first class meta programming support. This would nicely complement the Scheme gitignore.

[joeblau]

@willprice - This tempalte is live on gitignore.io - https://www.gitignore.io/api/racket

[sinistersnare]

I would like to bump this. It would be nice to Ctrl-F "racket" on this repo and get what i need.

[joeblau]

@sinistersnare Here are three other options:

1. go to https://github.com/dvcs/gitignore/tree/master/templates and ctrl+f there
2. https://www.gitignore.io and search for racket
3. install gitignore.io command line then run gi racket from your command line

[shiftkey]

@willprice apologies for the delay in replying to this.

I had a lot of these templates requested, but I couldn't quite figure out the right place for them. I ended up settling on #2857, and you'll see the Racket template can now be found at community/Racket.gitignore.

Feel free to submit any future improvements to that template, and thanks again for the contribution!