Readability improvements to README

[kylebarron]

Changes

• Refer to the "proj" convention by its proper name geo-proj, not proj:. proj: is the key prefix and geo-proj seems to be the proper name of the convention
• More hyperlinks. There should be easy links to the geo-proj and multiscales conventions when we reference them.
• Move field attributes from h4 to h3 to make their definitions more prominent
• Change several "bolded headers" to "h4 headers". This is important because it gives an explicit anchor to these headers. So a consumer can link directly to "coordinate convention", for example
• Move the section about "implications of using spatial:transform with multiscales" as part of the definition of spatial:transform, not the example section.

[kylebarron]

oops this slipped in from #15