Revert "Update README.rst"

This reverts commit 3892db35965346301703eefde74688ce7a9d88ca.
Revert "Warn about mixing Xcode and osx-gcc-installer"

This reverts commit 6bbc54253aef9b9dd773d48b753f50ea98284d9e.
Revert "Revert "Warn about mixing Xcode and osx-gcc-installer""

This reverts commit e6b2e5bb204ebec76d33a65a0788cb4923810e77.