explain README better

README.md

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-ConstraintTree provides a simple data structure `ConstraintTree` for organizing
-the contents of constraint-based linear problems. As a main goal, it abstracts
-over the distinction between constraints and variables, allowing much tidier
-representation for many kinds of complex constraint systems. The primary
-purpose is to support constraint-based metabolic modeling within COBREXA.jl.
+Package `ConstraintTrees.jl` provides a simple data structure `ConstraintTree`
+for organizing the contents of linear constrained optimization problems. As a
+main goal, it abstracts over the distinction between constraints and variables,
+allowing much tidier representation for many kinds of complex constraint
+systems. The primary purpose is to support constraint-based metabolic modeling
+within [COBREXA.jl](https://github.com/LCSB-BioCore/COBREXA.jl).
+
+`ConstraintTrees.jl` is new and under active development. Feel free to discuss
+the changes and ideas.
 
 #### Acknowledgements