Guide: add notes about bookmarks and markers.

guide/ch_tour.tex

@@ -264,6 +264,28 @@ \section{Taking Screenshots}
 \ref{sec:Directories} Files and Directories.
 
 
+\section{Bookmarks}
+\label{sec:tour:bookmarks}
+
+You can store your favourite objects or views with a bookmark. Press
+\key{Alt+B} or \guibutton[0.75]{2.5}{bt_bookmarks.png}. If date and/or
+location are relevant, make sure you activate the respective buttons.
+
+
+\section{Custom Markers}
+\label{sec:tour:markers}
+
+You can set temporary markers anywhere in the screen. These may be useful for various purposes but cannot be stored.
+
+\begin{center}
+\begin{tabular}{rl}\toprule
+\emph{Actions}	                 & \emph{Description}\\\midrule
+\key{Shift} \& left click        & Add custom marker \\
+\key{Shift} \& right click       & Delete the one closest marker to mouse cursor\\
+\key{Alt+Shift} \& right click   & Delete all custom markers 
+\\\bottomrule
+\end{tabular}
+\end{center}