Changed heading to Editing Modpack:

Changed heading to Editing Modpack from Edit Modpack because Edit
Modpack was confusing to newer users because it is the exact same text
as the Edit Modpack button.
Changed button to Save Editing

Changed the Edit Modpack button to Save Editing, because the button
saves your edits. This was a bit confusing to new users.
Added link to step by step instructions
Changed Save Editing to Save Changes

Changed Save Editing to Save Changes because Save Changes sounds a lot
better. Thanks sct.
Removed extra instructions

Removed extra instructions because they were incomplete/incorrect
Merge pull request #109 from Tommsy64/master

Changed some button names. Added link in README