Add 'when not to use curly braces' entry to style README

[ghubstan]

Added first entry to the style guide, explaining that devs
should not surround one line code blocks under if / else / while
statements with {curly braces}.

This patch was created in response to a comment in PR #3134
bisq-network/bisq#3134 (comment)

Since this is the first entry, and no protocol for asciidoc usage
has been defined, there will eventually need to be some discussion
about breaking up the readme into a table of contents linking to
sub-docs, and consistent usage of the asciidoc syntax.

[ManfredKarrer]

Actually we tended to use always brackets as it has caused already a few timed bugs in the past, specially if code got added later.

[ghubstan]

If it were up to me (only), I would also use brackets for the reasons stated above. I'm closing this PR and reverting the related commit back to using brackets around the one line block.