Document the metacompiling build process.

doc/build.md

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+This describes how lbForth is normally built.
+
+- To build a target, load its build.fth file.  Every target should
+  have one.
+
+- build.fth sets up some information about the target, and loads the
+  generic compile.fth.
+
+- compile.fth loads the generic metacompiler framework.
+
+  - The generic metacompiler loads a library for accessing a target image.
+
+  - And creates vocabularies for metacompiling words.
+
+- compile.fth proceeds to add metacompiling words that target lbForth.
+
+- And a target assembler.
+
+- It then loads kernel.fth and cold.fth to build a target image.
+
+- Finally, forward references are resolved, and the image is saved to
+  disk.