[bin][doc] add usage messages for README.md-generating scripts

bin/doc/cpp_to_md.sh

 #!/bin/sh
 
+# check if help is needed
+if test "$1" = "--help" || test "$1" = "-help" \
+   || test "$1" = "help" || test "$1" = ""; then
+  echo ""
+  echo "USAGE: $0 FILENAME"
+  echo ""
+  echo "The argument should be a C++ header or source file."
+  echo "The file is converted to Markdown and forwarded to stdout."
+  echo "In particular, a C++ comment is converted to its content,"
+  echo "while normal code is put into corresponding Markdown code blocks."
+  echo "Supposed to be called by the script merge_cpp_and_md.sh."
+  exit 0
+fi
+
 if [[ ${1: -3} == ".hh" ]]; then
   sed '1,/#define/d' $1 >tmpfile
   isheader=true

bin/doc/merge_cpp_and_md.sh

 #!/bin/sh
 
-# The arguments should be names of Markdown and/or C++ files.
-# The script loops through all arguments. If a Markdown file is encountered,
-# its content is forwarded to stdout. For another file, a Markdown header
-# stating the filename is printed and the script cpp_to_md.sh is applied.
+# check if help is needed
+if test "$1" = "--help" || test "$1" = "-help" \
+   || test "$1" = "help" || test "$1" = ""; then
+  echo ""
+  echo "USAGE: $0 FILENAME_1 [FILENAME_2 ...]"
+  echo ""
+  echo "The arguments should be names of Markdown and/or C++ files."
+  echo "The script loops through all arguments. If a Markdown file is encountered,"
+  echo "its content is forwarded to stdout. For another file, a Markdown header"
+  echo "stating the filename is printed and the script cpp_to_md.sh is applied."
+  echo "To be used for creating README.mds in the example folder. For example, by"
+  echo "bash $0 intro.md spatialparams.hh problem.hh main.cc results.md >README.md"
+  exit 0
+fi
 
 for filename in "$@"
 do