adds note on writing

README.md

@@ -49,6 +49,8 @@ Stuff that both works for leanpub and for bookdown:
 - Chapters can be referenced by using `[text of the link](#tag-of-the-title)`
 - Figures can be referenced by using `[text of the link](#fig:tag-of-r-chunk-that-produced-figure)`
 - Start and end mathematical expressions with `$` (inline) or with `$$` (extra line). Will be automatically changed for leanpub with a regexpr. Conversion script only works if no empty spaces are in the formula.
+- Leave empty lines between formulas and text (if formula not inline)
+
 
 ## Changelog
 All notable changes to the book will be documented here.