Documentation for marking entries

src/help/common.md

@@ -15,7 +15,20 @@ This help page is a work in progress.
 
 ## Marking entries
 
-TODO
+Sometimes it is convenient to perform an operation on multiple files at
+once: for example, copying multiple files. This is done by marking multiple
+files at once before performing the action. Mark or unmark a file by pressing
+**space**; pressing space multiple times will mark multiple files in
+sequence. Then press the key associated with the action to perform; for
+example, **Ctrl-C** to copy the marked files.
+
+Sometimes it can be tedious to mark each file one by one. When this is the
+case it is preferable to use the "mark pattern" command (**Ctrl-G**). This
+takes a glob-style wildcard pattern; for example, "*.png" or "*.wav" to
+match a set of files by extension, or "SPOS*" to match all lumps in a WAD
+with a particular prefix. To select every file, enter "*".
+
+To clear all marks, type **Ctrl-A** or **F10**.
 
 ## Searching