Update README

README.md

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 # Invistra: a portable and extensible Common Lisp FORMAT implementation
 
-The invistra system provides a portable implementation of Common Lisp FORMAT.
+The invistra system provides a portable implementation of Common Lisp
+FORMAT. It was initially part of [SICL][] but has been extracted and
+the missing required features completed.
+
+# Requirements
+
+[ABCL][], [Clasp][], [ECL][], and [SBCL][] is the only current
+implementations that this system has been tested on. 
+
+In addition to a clone of this repository in a location that is
+discoverable by ASDF you will also need a clone of
+[trivial-stream-column][], [Incless][] and [Inravina][].
+
+# Usage
+
+The core functionality is in the `invistra` package, but the Common
+Lisp-like interface of `format` and `formmatter` is in the
+`invistra-extrinsic` package and system. To call format try
+the following in SBCL:
+
+```
+* (asdf:load-system :invistra-extrinsic)
+T
+* (invistra-extrinsic:format t "Wibble: ~a~%"
+                             :quux)
+Wibble: QUUX
+NIL
+```
+
+Invistra can also replace an implementation's FORMAT
+implementation. This is done with the `invistra-shim` system.
+
+```
+* (asdf:load-system :invistra-shim)
+T
+* (format t "Wibble: ~a~%" :quux)
+Wibble: QUUX
+NIL
+```
+
+[ABCL]: https://armedbear.common-lisp.dev/
+[CLASP]: https://github.com/clasp-developers/clasp
+[ECL]: https://ecl.common-lisp.dev/
+[Incless]: https://github.com/s-expressionists/Incless
+[Inravina]: https://github.com/s-expressionists/Inravina
+[SBCL]: http://sbcl.org
+[SICL]: https://github.com/robert-strandh/SICL
+[trivial-stream-column]: https://github.com/yitzchak/trivial-stream-column