README, test: additional documentation

README

@@ -57,3 +57,15 @@ a horizontal tab, which is removed from the argument; if a line in the command
 section is not preceded by a tab, the omission is considered a syntax error (a
 "naked line") and menu(1) exits unsuccessfully. The command section ends with
 two consecutive newlines.
+
+## POSIX systems
+
+The Makefile and example shell scripts should work fine with POSIX make(1) and
+sh(1). Other POSIX.1 utilities are used within the limits of the POSIX
+specification. ytfeed(6) additionally requires python3(1) to be present in the
+PATH; this should be a Python 3 interpreter.
+
+## Non-POSIX systems
+
+Depending on how foreign the system is, some or much work may be required to
+port menu(1) and its examples.

test

@@ -8,6 +8,10 @@ $ cat Makefile
 
 	cat Makefile
 
+$ less <menu.rs
+
+	less <menu.rs
+
 Remove menu and re-build it.
 
 	rm -f menu; make