Added parsing documentation

docs/parsing.md

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+# Parsing HTML5 sites
+
+Parsing a HTML5 site is not difficult, although it currently require some manual work. Later on, this will be encapsulated in the engine API.
+
+First, we need to fetch the actual HTML content. This can be done by a simple HTTP request, or reading a file from disk. These HTML bytes must be 
+passed to the char streamer:
+
+```rust
+
+    let mut chars = CharIterator::new();
+    chars.read_from_str(&html, Some(Encoding::UTF8));
+```
+
+Here, the &html points to a string containing the HTML content. The `CharIterator` will take care of converting the bytes to characters, and handle the encoding.
+We assume UTF-8 here, but other encodings could be supported later on as well.
+
+
+Next, we need to create a document, which will be the main object that will be filled by the parser. The document will contain all the node elements and other 
+data that is generated during the parsing of the HTML. This also includes any stylesheets that are found, both internally and externally.
+
+```rust
+    let document = DocumentBuilder::new_document();
+```
+
+Note that a document itself isn't a document, but a HANDLE to a document (a `DocumentHandle`). Once we have our document handle, we can start the parser
+by calling the `parse_document` method on the `Html5Parser` struct. This method will return a list of parse errors, if any. 
+
+```rust
+    let parse_errors = Html5Parser::parse_document(&mut chars, Document::clone(&document), None)?;
+
+    for e in parse_errors {
+        println!("Parse Error: {}", e.message);
+    }
+```
+
+If there are any errors during parsing, they will be added to the parse_errors list. These errors can be printed to the console, or handled in any other way.
+
+Finally, we can do whatever we need to do with the document. Normally it will be used to render the HTML by passing it into a render pipeline, but for now
+we can simply print the document. This will output a tree-like structure of all node and text elements found in the document.
+
+```rust
+    println!("Generated tree: \n\n {document}");
+```
+
+It is possible to traverse the document tree with a visitor pattern. You can create a visitor struct that implements the `NodeVisitor` trait, and then pass
+it to the `visit` method.
+
+```rust
+    let mut visitor = Box::new(TextVisitor::default()) as Box<dyn Visitor<Node>>;
+    visit(&Document::clone(&document), &mut visitor);
+```
+
+A simple visitor could hide all non-renderable nodes, or change the text color based on the CSS properties, or even generate colored links for `<a>` tags.