README: Re-wrap and improve formatting

[waldyrious]

• Mention the abbreviation "GAE" next to the first occurrence of the expanded expression (this one comes from #179)
• Wrap consistently at 80 characters (there were various limits used in different parts of the document)
• Fix multi-line list item (subsequent lines had to be indented to be considered part of the same list item)
• Add missing code formatting with backticks
• Consistently use double backticks for inline code
• Consistently use two blank lines above section headers
• Fix a typo ("int the" → "in the")
• Fix capitalization in mid-sentence parenthetical
• Consistently indent code blocks with 4 space characters
• Remove space before :: to match the dominant style in the document

[Abhishek-IOT]

The check might not have been completed because of the docker limit has reached. https://github.com/sympy/sympy-live/issues/188