Update README.md

Add a note about installing hoedown via CocoaPods.
Merge pull request #67 from clemensg/master

Add a note about CocoaPods to the README
Fix implicit conversion loss compiler warning in parse_codefence
Merge pull request #70 from clemensg/fix_conversion_loss_warning

Fix implicit conversion loss compiler warning in parse_codefence
Improve hoedowns gitignore file for Mac users.
Merge pull request #71 from clemensg/gitignore_mac

Improve hoedowns gitignore file for Mac users
Talk about client-side attacks