Clarifying definition of Open Source and other changes

[andrewjskatz]

Now:

• explains why OpenChain uses a different definition of open source to the OSI
• Changes the patent wording slightly to refer to explicit patent licensing (so acknowledging that open source licences may include implied licences)
• generally refers to license conditions as opposed to obligations
• Refers to deemed distribution licenses like AGPL
• Avoids implying that MPL-2.0 has a modification notice
• Other minor typos and formatting (e.g. correcting numbered list)

Thanks to the Japan Work Group for their great work on this