docs: update README and OPAM descriptions

Author: favonia

README.markdown

@@ -1,6 +1,6 @@
 # ♾️ mugen 無限: Universe Levels
 
-A vast generalization of [Conor McBride’s crude but effective stratification](https://personal.cis.strath.ac.uk/conor.mcbride/Crude.pdf). We replace natural numbers with what we call _displacement algebras_, the minimum algebraic structure to use McBride’s scheme. Our (conditionally accepted) POPL 2023 paper [“An Order-Theoretic Analysis of Universe Polymorphism”](https://favonia.org/files/mugen.pdf) explained the theory behind our design.
+A vast generalization of [Conor McBride’s crude but effective stratification](https://personal.cis.strath.ac.uk/conor.mcbride/Crude.pdf). We replace natural numbers with what we call _displacement algebras_, the minimum algebraic structure to use McBride’s scheme. Our POPL 2023 paper [“An Order-Theoretic Analysis of Universe Polymorphism”](https://favonia.org/files/mugen.pdf) explained the theory behind our design.
 
 ## Stability
 

mugen.opam

@@ -1,7 +1,9 @@
 opam-version: "2.0"
 name: "mugen"
-synopsis: "Universes"
-description: """Universe Levels"""
+synopsis: "Universe levels and universe polymorphism"
+description: """
+This package implements the generalization of Conor McBride’s crude but effective stratification. The theory behind the design is discussed in the POPL 2023 paper “An Order-Theoretic Analysis of Universe Polymorphism.”
+"""
 maintainer: "favonia <favonia@gmail.com>"
 authors: "The RedPRL Development Team"
 license: "Apache-2.0"