Updates README based on usage section

README.md

@@ -24,8 +24,12 @@ We can access values by dot-separated addresses. For instance, we can
 use `courses.datintro22.schedule.url` to access the TimeEdit URL of the
 datintro22 course.
 
-Let's have a look at some usage examples. Say we have the program
-`nytid` that wants to use this config module and subcommand.
+Let's have a look at some usage examples.
+
+### A command-line application
+
+Say we have the program `nytid` that wants to use this config module and
+subcommand.
 
 ```python
 import typer
@@ -59,3 +63,54 @@ import typerconf as config
 
 url = config.get("courses.datintro22.schedule.url")
 ```
+
+### Without the CLI
+
+We can also use it without the CLI and application features. Then it's
+the `typerconf.Config` class that is of interest.
+
+Let's assume that we have the structure from above in the file 
+`~/.config/app.config`. Consider the following code.
+
+```python
+defaults = {
+  "courses": {
+    "datintro22": {
+      "root": "/afs/kth.se/..."
+    }
+  }
+}
+
+conf = Config(json_data=defaults, conf_file="~/.config/app.config")
+
+print(f"datintro22 root directory = {conf.get('courses.datintro22.root')}")
+print(f"datintro22 schedule = {conf.get('courses.datintro22.schedule')}")
+```
+
+When we construct `conf` above, we merge the default config
+with the values set in the config file that was loaded.
+
+We note that the construction of `conf` above, can be replaced
+by the equivalent
+
+```python
+conf = Config(defaults)
+conf.read_config("~/.config/app.config", writeback=True)
+```
+
+The meaning of `writeback` is that whenever we change the
+config, it will automatically be written back to the file from which it
+was read. Writeback is enabled by default when supplying the file to the
+constructor. It's disabled by default for the method
+`conf.read_config`, but we can turn it on by passing the
+`writeback=True`.
+
+We can change the config by using the `conf.set` method.
+
+```python
+conf.set("courses.datintro22.root", "/home/dbosk/...")
+```
+
+That would change the root that we got from the default config. Since we
+enabled writeback above, this would automatically update the config file
+with the new values.