How could I add a note ... or a footnote at the beginning of the Table of Contents, something like 'Chapters with * can be skipped on a first reading'...?

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You can use \addtocontents to add whatever you like (*) to the .toc file that is later read in by \tableofcontents. (*) With the caveat that you adequately \protect any fragile commands you might wish to include.

Something like this might work for you:

\documentclass[oneside]{book}
\usepackage{lipsum}

\begin{document}
\frontmatter
\tableofcontents
\mainmatter
\chapter{One}
\lipsum[1-10]
\chapter[Two (*)]{Two}
\lipsum[1-10]
\chapter{Three}
\lipsum[1-10]
\addtocontents{toc}{\bigskip\noindent Note: chapters with * can be skipped on a first reading.}
\end{document}


The .toc file it produces looks like this:

\contentsline {chapter}{\numberline {1}One}{1}{chapter.1}
\contentsline {chapter}{\numberline {2}Two (*)}{4}{chapter.2}
\contentsline {chapter}{\numberline {3}Three}{7}{chapter.3}
\vspace \bigskipamount \noindent Note: chapters with * can be skipped on a first reading.


which eventually leads to output that looks like this:

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