Added pydocs and updated the README

[pendingintent]

README.md — substantially expanded with new sections covering:

• Loading ODM-XML, ODM-JSON, Define-XML, and ARM documents
• Loading from strings
• Creating documents programmatically (bottom-up construction)
• Fluent builder (ODMBuilder) with a realistic multi-element example
• Context managers (open_odm, open_define) for read-modify-write workflows
• Serialization (XML string, JSON string, Python dict, file output)
• All four validation layers: OID ref/def, Cerberus conformance, XSD schema, combined error collection
• Element ordering (verify_order / reorder_object)
• OID index lookups (build_oid_index)
• find, find_all, find_by helpers
• Dataset-JSON: creation, XML↔JSON conversion, Define-XML flattening
• Pandas integration (metadata_to_dataframe, clinical_data_to_dataframe, define_metadata_to_dataframes, etc.)
• Namespace management

Pydocs added to three modules that were missing them:

• document_loader.py — module docstring + docstrings on all abstract methods in DocumentLoader
• odm_parser.py — module docstring + full docstrings on SchemaValidator, ODMSchemaValidator, BaseParser, ElementParser, ODMParser, ODMStringParser,
  and ODMJSONStringParser
• oid_index.py — module docstring with usage example + class and method docstrings on OIDIndex