docs: add info about available features in the root README

Author: ksachdeva

README.md

@@ -1,3 +1,11 @@
 # Devcontainer features
 
 Repository for various custom devcontainer features
+
+## Available features
+
+| Feature     | Description                 | Documentation                        |
+|-------------|-----------------------------|--------------------------------------|
+| nats-server | NATS messaging server       | [README](./src/nats-server/README.md)    |
+| nats-top    | NATS server monitoring tool | [README](./src/nats-top/README.md)       |
+| natscli     | NATS command-line client    | [README](./src/natscli/README.md)        |