Refine instructions for checking out sources.

For those looking for a direct way to check out sources without
needing to upload SSH keys to GitHub, provide checkouts using https.

For those who likely will be doing direct commits and have their
SSH keys uploaded to GitHub, provide URLs for cloning over SSH.

Author: tkremenek

README.md

@@ -66,6 +66,22 @@ compiler for C++14 support and create a symlink:
 
 ### Getting Sources for Swift and Related Projects
 
+	For those checking out sources as read-only:
+
+     git clone https://github.com/apple/swift.git swift
+     git clone https://github.com/apple/swift-llvm.git llvm
+     git clone https://github.com/apple/swift-clang.git clang
+     git clone https://github.com/apple/swift-lldb.git lldb
+     git clone https://github.com/apple/swift-cmark.git cmark
+     git clone https://github.com/apple/swift-llbuild.git llbuild
+     git clone https://github.com/apple/swift-package-manager.git swiftpm
+     git clone https://github.com/apple/swift-corelibs-xctest.git
+     git clone https://github.com/apple/swift-corelibs-foundation.git
+
+	For those who plan on regular making direct commits, cloning over
+	SSH may provide a better experience (which requires uploading 
+	SSH keys to GitHub):
+
      git clone [email protected]:apple/swift.git swift
      git clone [email protected]:apple/swift-llvm.git llvm
      git clone [email protected]:apple/swift-clang.git clang
@@ -76,7 +92,6 @@ compiler for C++14 support and create a symlink:
      git clone [email protected]:apple/swift-corelibs-xctest.git
      git clone [email protected]:apple/swift-corelibs-foundation.git
 
-
 [CMake](http://cmake.org) is the core infrastructure used to configure builds of
 Swift and its companion projects; at least version 2.8.12.2 is required. Your
 favorite Linux distribution likely already has a CMake package you can install.