Add intro inside of src/ directory

src/README.md

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+## Welcome to the Babel Codebase
+
+Babel is broken down into three parts:
+
+- Parsing
+- Transformation
+- Generation
+
+**Parsing** is the process of turning a piece of code into an AST (Abstract
+Syntax Tree) which is a tree-like object of nodes that _describes_ the code.
+This makes it easier to transform the code over direct string manpulation.
+
+**Transformation** is the process of taking an AST and manipulating it into
+a new one. For example, Babel might take an ES2015 ArrowFunction and transform
+it into a normal Function which will work in ES5 environments.
+
+**Generation** is the process of turning an AST back into code. After Babel is
+done transforming the AST, the generation takes over to return normal code.
+
+---
+
+Babel's parsing step is done by Acorn. However, because Babel is implementing
+future standards it maintains a fork of Acorn in order to do it's job. You can
+see that fork in the "acorn" folder.
+
+The transformation and generation steps are both handled inside of the Babel
+codebase itself, which is in the "babel" folder here.