Update README.md

Added installation instructions
Update README.md
Update README.md
Update README.md
Update README.md

Added header to install section
Merge branch 'patch-7' of https://github.com/amuramoto/apigee-ios-sdk into amuramoto-patch-7

Conflicts:
	README.md
Merge branch 'master' of https://github.com/apigee/apigee-ios-sdk