Fix grammar and use correct markdown in readme.

[samuelh2005]

This PR includes the following:

• improved grammar in some places
• headings being used as they are supposed to be (diving the document into sections - not to highlight important information)
• use correct level of headings in the right order (for example level 4 headings (####) should only come after a level 3 heading (###))
• fix closing of some code blocks (before they used ''' instead of ``` which causes the headings to be included as code.
• use bold, italics, and "alert boxes" to highlight information.
• use accessible URL for UNMS docs.
• improve + add some some links.