add readme

Author: elkowar

README.md

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+# simplexpr
+
+simplexpr is a parser and interpreter for a simple expression syntax that can be embedded into other applications or crates.
+It is being developed to be used in [eww](https://github.com/elkowar/eww), but may also other uses.
+
+For now, this is highly experimental, unstable, and ugly. You most definitely do not want to use this crate.