Add sections to README.

README.md

@@ -18,11 +18,21 @@ The project has a threefold aim:
 I want to stress the fact that this is not a generically-compliant POSIX shell project; on the contrary, the aim of the project is to write code that will run on (and require) Bash. As to the rest of the environment, however, the long-term goal is to get rid of dependencies on external tools as far as it is possible (or reasonable) to do so, even though at present the code does assume a reasonably standard GNU/Linux system.
 
 
+## Rationale
+
+TODO
+
+
 ## Contributing
 
 Of course there is so much work to do, and I'd be delighted if somebody is willing to step in with code, enhancements, suggestions, and of course test cases and bug reports!
 
 
+## Requirements
+
+TODO
+
+
 ## Installing
 
 A reasonably sane GNU/Linux environment is assumed to be at hand; support for FreeBSD and Cygwin is available (in _homeopathic dilution_), yet highly experimental.