Prep for next release

[macblazer]

• Update the README.md file with a link to the reference documentation at https://engineering.jamf.com/Subprocess/documentation/subprocess/.
• Update the CHANGELOG file with all non-trivial changes.
• Update the Subprocess.podspec file with the new version number.

After the merge:

• Put a git tag on the next version.
• Update public CocoaPods listing.