Add a markdown style guide

[dyladan]

Related to w3c/trace-context#485

[basti1302]

Some other things that come to mind:

• We could link to https://www.w3.org/Guide/manual-of-style/ at the start of the style guide. Suggestion: "In general, the W3C Manual of Style applies. The following style guide extends it with additional rules."
• In addition to the comment on how to reference fields, should we also include a comment on kebab case (span-id vs span ID vs SpanId)? Suggestion:

Naming fields

Field names are generally lower case. Fields names consisting of multiple words should be written in kebab case (e.g. span-id). Use code markup (markdown backticks) when referring to field names. Do not use code markup or kebab case when referring to a general concept instead of a specific field name. (e.g. "This is the ID of the operation of the callee (in some systems known as the span ID) and is used to uniquely identify an operation within a trace.")

[basti1302]

@dyladan this has two approving reviews and has been open for a while, I think we could go ahead and merge, WDYT?

There is one pending open question from me but it is not blocking and the suggestions could also be applied later (or never, that's fine, too).