Flesh out README

README

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-GNU lightning is a library to aid in making portable programs
-that compile assembly code at run time.  For more information,
-look at the info documentation.

README.md

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+# Lightening
+
+Lightening is a just-in-time code generation library derived from GNU
+Lightning, adapted to the purposes of the GNU Guile project.
+
+## Use
+
+```
+gcc -flto -O2 -g -o jit.o -c jit/jit.c
+gcc -flto -O2 -g -o my-program jit.o my-program.c
+```
+
+See the GNU Lightning manual for more on how to program against
+Lightening (much of the details are the same).
+
+## What's the difference with GNU Lightning?
+
+This project is called Lightening because it's lighter-weight than GNU
+Lightning.  When you go to generate code at run-time with GNU Lightning,
+what happens is that you build up a graph of nodes which GNU Lightning
+"optimizes" before finally emitting machine code.  These optimizations
+can improve register allocation around call sites.  However they are not
+helpful from a Guile perspective, as they get in the way of register
+allocation that we need to do; and they actually prevent access to all
+the registers that we would like to have.
+
+Guile needs a simple, light-weight code generation library.  The GNU
+Lightning architecture-specific backends provide the bulk of this
+functionality, and Lightening wraps it all in a lightweight API.