Bootstrap README

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+Bootstrapping chain for BQN. The following gets you a full compiler:
+
+- Files boot2.bqn and boot1.bqn here, and ../c.bqn
+- A compiler capable of building boot2.bqn
+- Primitive support sufficient to run all of the above
+
+Each compiler can compile the previous one, but not ones before that.
+Run verify.bqn to check that object code produced at each step matches
+the full compiler's output.
+
+The compiler boot2.bqn uses an easy-to-compile subset of BQN syntax.
+Syntax restrictions for the full chain are listed below.
+
+Full compiler, boot 0
+  No headers or namespaces (or decimals) . :;? ⇐
+  No high-rank array syntax []
+  No useless parentheses that enclose a single value
+
+Boot -1
+  No stranding ‿ or character literals '
+  No " in comment: comments can safely be parsed before strings
+  No _: token role determined by first character
+  No Nothing: · doesn't appear and 𝕨 always has a value
+  Only 𝕊𝕩𝕨 for special names
+
+Boot -2
+  Single-scope
+  No modified or list assignment
+  No 2-modifiers at all
+
+Boot -3
+  Not easier to compile, just cut down further