docs(readme): add readme

README.md

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+# mcl
+a math command line.
+
+## features
+currently supports:
+- lossy/non-exact math (will not store fractions)
+- basic math functions (e.g. sqrt, normal + inverse trig functions, abs)
+- variables (currently do basically nothing)
+
+planned features:
+- exact math (radicals, fractions, etc.)
+- solving equations
+- a custom math language
+  - scripting support
+  - user defined functions
+will most of these features ever actually be added? probably not. but wishful thinking is important!
+
+## usage
+you can evaluate math in a couple ways while getting the result automatically:
+```
+$ mcl eval "1 + 1"
+$ echo "1 + 1" | mcl
+```
+
+you can also use the repl to evaluate math:
+```
+$ mcl
+> 1 + 1
+2
+```
+
+for full usage, see `mcl -h`