Document basic usage of bcf library

Author: Moult

src/bcf/README.md

@@ -5,3 +5,55 @@ Manipulation of BCF-XML is available via `bcfxml.py` and manipulation of BCF-API
 is available via `bcfapi.py`.
 
 Currently supports BCF version 2.1.
+
+## bcfxml
+
+The `bcfxml` module lets you interact with the BCF-XML standard.
+
+```
+from bcf.bcfxml import BcfXml
+
+bcfxml = BcfXml()
+
+# Load a project
+project = bcfxml.get_project("/path/to/file.bcf")
+
+# The project is also stored in the module
+# project == bcfxml.project
+
+print(project.name)
+
+# To edit a project, just modify the object directly
+bcfxml.project.name = "New name"
+bcfxml.edit_project()
+
+# The BCF file is extracted to this temporary directory
+print(bcfxml.filepath)
+
+# Get a dictionary of topics
+topics = bcfxml.get_topics()
+
+# Note: topics == bcfxml.topics
+for guid, topic in bcfxml.topics.items():
+    print("Topic guid is", guid)
+    print("Topic guid is", topic.guid)
+    print("Topic title is", topic.title)
+
+    # Fetch extra data about a topic
+    header = bcfxml.get_header(guid)
+    comments = bcfxml.get_comments(guid)
+    viewpoints = bcfxml.get_viewpoints(guid)
+
+    # Note: comments == topic.comments, and so on
+    for comment_guid, comment in comments.items():
+        print(comment_guid)
+        print(comment.comment)
+        print(comment.author)
+
+# Get a particular topic
+topic = bcfxml.get_topic(guid)
+
+# Modify a topic
+topic.title = "New title"
+bcfxml.edit_topic(topic)
+```