Added README

README.md

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+# Pycalc
+
+**Pycalc** is a simple expression calculator. It can evaluate mathematical expressions, and output the result of the evaluation.
+
+## How it works
+1. Scan the expression
+	- Check for malformed expression and invalid characters
+	- Tokenize the expression (I wrote a custom parser to do this)
+  2. If there are any parantheses, replace the parantheses with the result of the sub-expression.
+  3. Evaluate the overall expression (which should no longer have any parantheses)
+
+Rather than building a tree from the expression (which is the recommended approach), I have opted to perform the substitutions in-place. This means that the result of an operation replaces the operation itself. The reason I chose to do this, is that my parsing algorithm considers everything inside parantheses as a single token. Therefore, it is fairly easy to replace this token with the result of the expression within the parantheses.
+
+## Features
+- Order of operations
+- Arbitrary whitespace
+- Parantheses
+- Simple error checking
+
+## To-do
+- Simple algebraic parsing (assign values to variables, etc.)
+- More robust error-checking