Add README with basic usage information

my2to3/README.md

+# My2to3
+
+This documentation is rudimentary.
+
+## Apply "trace" fixers on the fly
+
+* When importing modules:
+  
+  ```python
+  from my2to3.trace import patch_imports
+  
+  def is_whitelisted(fullname, path):
+    # This function is used to whitelist specific modules. This corresponds to
+    # https://www.python.org/dev/peps/pep-0302/.
+    return True
+  
+  patch_imports(is_whitelisted)
+  ```
+
+* On more complex components (i.e. with monkey-patching)
+  
+  ```python
+  from my2to3.trace import tracing_functions, apply_fixers
+  
+  # `tracing_function` contains all the functions registered with
+  # `@register_tracing_function`. Add its content to the builtins (or
+  # equivalent) of the component to monkey-patch, e.g.:
+  component_builtins.update({f.__name__: f for f in tracing_functions})
+  
+  # `name` must be a unique name which represents the code to modify, to easily
+  # recognize it. In general, we use a filepath (or an equivalent). This is used
+  # by the fixers.
+  component_code = apply_fixers(component_code, name)
+  ```