adds more text about markdowns to Writing Guidebook Entries

[UBlueberry]

Two reasons for this:
A: General knowledge so everyone knows what markdown tags there are and how to use them
B: To standardize markdown tag usage on the Wizden guidebook

[juliangiebel]

Please don't call it markdown. The tags like this: [color ...] are called richtext or bbcode. The # is from guidebook code and is hacked in "markdown syntax"

[UBlueberry]

whoopsie!