Add .gitignore files

[ewels]

Having a .gitignore file is important - it prevents people from accidentally committing files to the git repository (I'm looking at you, OSX .DS_Store 👀 ).

.gitignore files are typically language-specific, so a different file will be needed for each repo. GitHub can add these files to new repos for you when you create them through GitHub (when using "initialise with a README option). You can find all of the GitHub template .gitignore files here: https://github.com/github/gitignore