Update clusters.md (#619)

added a table and some high level info on clusters, if you could double check that I used the right endpoints in the table, if you decide it's helpful,  that would be so great

docs/core/clusters.md

@@ -39,6 +39,21 @@ An example of some of these Solana blockchain explorers include:
 - [http://solanabeach.io/](http://solanabeach.io/).
 - [http://validators.app/](http://validators.app/).
 
+## On a high level
+
+- Mainnet: Live production environment for deployed applications.
+- Devnet: Testing with public accessibility for developers experimenting with their applications.
+- Testnet: Stress-testing for network upgrades and validator performance.
+
+**Example use cases**: You may want to debug a new program on Devnet or verify performance metrics on Testnet before Mainnet deployment.
+
+| **Cluster**   | **Endpoint**                     | **Purpose**                                 | **Notes**                          |
+|---------------|----------------------------------|---------------------------------------------|-------------------------------------|
+| Mainnet       | `https://api.mainnet-beta.solana.com` | Live production environment                 | Requires SOL for transactions      |
+| Devnet        | `https://api.devnet.solana.com`      | Public testing and development              | Free SOL airdrop for testing       |
+| Testnet       | `https://api.testnet.solana.com`     | Validator and stress testing                | May have intermittent downtime     |
+
+
 ## Devnet
 
 Devnet serves as a playground for anyone who wants to take Solana for a test