Add draft for comparison

src/haasdaiga/constructions/comparison.md

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+# Adjectives and comparison
+
+## Adjectival verbs
+
+Some verbs play the function adjectives do in other languages.  Such verbs
+are called *adjectival verbs*.
+
+In the predicative form, it is used in the same way as other verb.
+
+> man-nom.mt.sg is_old-ind.prs.cnt  
+> *The man is old.*
+
+In the descriptive form, it is attached before the noun without any
+conjugation.
+
+> is_old-man-nom.mt.sg catch-ind.pst.cnt fish-acc.wa.sg
+> The old man caught the fish
+
+## Comparison
+
+Comparison is done via prefixing.
+
+### Comparative form
+
+The comparative form has prefix **jìs**.
+
+### Superlative form
+
+The superlative form has prefix **haät**.