Documented code generators

internal/elementgen/README.md

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+# elementgen
+
+elementgen reads simple descriptions of HTML elements from `elements.txt`
+and generates Go functions to create them. The file contains one description
+per line. Each description is in the form
+```
+tag-name element-type
+```
+where `tag-name` is the lowercase name of the element, and `element-type` is
+either `empty` for empty elements (e.g. `img`, 'br') or `parent` for elements
+that can have child nodes (e.g. 'div', 'body').