Add a SQL support documentation

[Bexanderthebex]

Why? 🤔

• There is SQL support for marketstore, though limited, should be documented in order to let other developers know what is the current state of it

What? 🗒️

• Adds Markdown documentation of the current state of the SQL support for MarketStore

[codecov-commenter]

Codecov Report

Merging #522 (37fb44c) into master (b1b7186) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   47.81%   47.81%           
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  Files         125      125           
  Lines       12376    12376           
=======================================
  Hits         5918     5918           
  Misses       5959     5959           
  Partials      499      499