add a CONTRIBUTING.md file

add a CONTRIBUTING.md file, copied verbatim from the "Pull Requests" section in README.md
Ignore Vim untitled and unsaved buffer swapfiles

Vim creates buffer swapfiles starting at .swp for buffers that are
active but untitled and unsaved.
Remove Podfile.lock from Objective-C ignores

Per discussion in #148.
Merge pull request #493 from strugee/master

This repository didn't have a CONTRIBUTING.md file. Now it does.
Merge pull request #576 from nysa/master

Ignore Vim untitled and unsaved buffer swapfiles