Updated README.md to document template system

utils/build_help_table.py

@@ -0,0 +1,8 @@
+""" Builds the template table in markdown format for README.md """
+
+from osxphotos.template import TEMPLATE_SUBSTITUTIONS
+
+print("| Substitution | Description |")
+print("|--------------|-------------|")
+for subst, descr in TEMPLATE_SUBSTITUTIONS.items():
+    print(f"|{subst}|{descr}|")