Add subsections to Understanding Internals

DEVELOPMENT.md

@@ -41,7 +41,8 @@ code accordingly. We also use [clippy](https://github.com/rust-lang/rust-clippy)
 to detect rust code issues. 
 
 Python code should follow the 
-[PEP 8](https://www.python.org/dev/peps/pep-0008/) style.
+[PEP 8](https://www.python.org/dev/peps/pep-0008/) style. We also use
+[flake8](http://flake8.pycqa.org/en/latest/) to check Python code style.
 
 ## Testing
 
@@ -107,22 +108,46 @@ repository's structure:
 
 ## Understanding Internals
 
-Rust crates
-  rustpython: top level crate
-  rustpython_parser
-  rustpython-vm (source is found in `vm/src`)
+The RustPython package is the `rustpython` top-level crate. The `Cargo.toml`
+file in the root of the repo provide configuration of the crate and the
+implementation is found in the `src` directory (specifically, 
+`src/main.rs`).
 
-Lexer, Parser, AST (Abstract Syntax Tree)
+The top-level `rustpython` depends on several lower-level crates including:
+
+- `rustpython-parser` (source: `parser/src`)
+- `rustpython-compiler` (source:`compiler/src`) 
+- `rustpython-vm` (source: `vm/src`)
+
+Together, these crates provide the functions of a programming language and
+enable a line of code to go through a series of steps:
+
+- parse the line of source code into tokens
+- determine if the tokens are valid syntax
+- create an Abstract Syntax Tree (AST)
+- compile the AST into bytecode
+- execute the bytecode in the virtual machine (VM).
+
+### rustpython-parser
+
+This crate contains the lexer and parser to convert a line of code to
+an Abstract Syntax Tree (AST):
 - Lexer: `parser/lexer.rs` converts Python source code into tokens
 - Parser: Takes the tokens generated by the lexer and parses the tokens
   into an AST (Abstract Syntax Tree) where the nodes of the syntax tree
-  are Rust structs and enums
+  are Rust structs and enums.
   - The Parser relies on `LALRPOP`, a Rust parser generator framework
   - More information on parsers and a tutorial can be found in the 
     [LALRPOP book](https://lalrpop.github.io/lalrpop/README.html).
+- AST: `parser/ast.rs` implements Python types and expressions
+  represented in by the AST nodes in Rust.
+
+### rustpython-compiler
 
-Compiler
 Generates bytecode from the AST
+(source is found in `compiler/src`) is primarily used to transform AST (Abstract Syntax Tree to bytecode)
+
+### rustpython-vm
 
 VM (Virtual Machine)
 Fetch and dispatch loop