[chore][receiver/journald]: improve README.md examples (#42692)

#### Description
Add the following examples to the documentation:
 - Minimal configuration
 - Cursor tracking
 - Reading from the beginning

<!--Describe the documentation added.-->
#### Documentation
This PR improves the documented examples

---------

Co-authored-by: Andrzej Stencel <[email protected]>

Author: belimawr

receiver/journaldreceiver/README.md

@@ -56,6 +56,49 @@ Each operator performs a simple responsibility, such as parsing a timestamp or J
 - Only parsers and general purpose operators should be used.
 
 ### Example Configurations
+#### Minimal configuration
+
+The following configuration is the minimal configuration to read
+journald logs:
+
+```yaml
+receivers:
+  journald:
+```
+will be passed to journalctl as the following arguments: `journalctl
+... --priority info`. This will read the 10 most recent entries and
+any subsequent entry.  `--priority info` is the default priority, the
+following examples will omit it for simplicity.
+
+#### Cursor tracking
+```yaml
+receivers:
+  journald:
+    storage: file_storage/journald
+
+extensions:
+  file_storage/journald:
+    directory: .
+
+service:
+  extensions: [file_storage/journald]
+```
+
+If you stop and start the otel collector, only new entries will be
+read.
+
+#### Reading from the beginning
+
+```yaml
+receivers:
+  journald:
+    start_at: beginning
+```
+
+will be passed to journalctl as the following arguments: `journalctl
+... --no-tail`. This will read all messages from the current boot.
+
+#### Units
 
 ```yaml
 receivers: