Add TODOTXT_VERBOSE to the configuration

There's no command-line option to reduce verbosity (just -v to increase it), so users who would like to remove the additional messages (cp. https://github.com/todotxt/todo.txt-cli/discussions/364) have to configure this, but the variable is hard to find.
Include the default value in commented-out form and some documentation of the possible values.

todo.cfg

@@ -76,6 +76,13 @@ export REPORT_FILE="$TODO_DIR/report.txt"
 
 # === BEHAVIOR ===
 
+## verbosity
+#
+# By default, additional information and confirmation of actions (like
+# "TODO: 1 added") are printed. You can suppress this via 0 or add extra
+# verbosity via 2.
+# export TODOTXT_VERBOSE=1
+
 ## customize list output
 #
 # TODOTXT_SORT_COMMAND will filter after line numbers are