feat: add README

Signed-off-by: chenchanglew <lewchenchang@gmail.com>

samples/chaincode/secret-keeper-go/README.md

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+# Secret Keeper
+
+Secret Keeper is a demo application designed to securely store sensitive information, acting as a digital vault. It's ideal for users who need to manage access to shared secrets within a team or organization.
+
+## Functions
+
+Secret Keeper provides the following functionalities:
+
+- **InitSecretKeeper**: Initializes the application with default authorization and secret values. Intended for one-time use at application setup. Note: While potential misuse is considered low-risk, it's recommended to secure access to this function.
+
+- **RevealSecret**: Allows authorized users to view the currently stored secret.
+
+- **LockSecret**: Enables authorized users to update the secret value. This action replaces the existing secret.
+
+- **AddUser**: Permits authorized users to add a new user to the authorization list, granting them access to all functions.
+
+- **RemoveUser**: Allows authorized users to remove an existing user from the authorization list, revoking their access.
+
+## Example Usage
+
+To demonstrate Secret Keeper's capabilities, you can deploy the chaincode to [the-simple-testing-network](https://github.com/hyperledger/fabric-private-chaincode/tree/main/samples/deployment/fabric-smart-client/the-simple-testing-network) and then invoke it with the [simple-cli-go](https://github.com/hyperledger/fabric-private-chaincode/tree/main/samples/application/simple-cli-go).
+
+1. Initialize Secret Keeper:
+```
+./fpcclient invoke initSecretKeeper
+```
+2. Reveal the secret as Alice:
+```
+./fpcclient query revealSecret Alice
+```
+3. Change the secret as Bob:
+```
+./fpcclient invoke lockSecret Bob NewSecret
+```
+4. Attempt to reveal the secret as Alice (now updated):
+```
+./fpcclient query revealSecret Alice
+```
+5. Remove Bob's access as Alice:
+```
+./fpcclient invoke removeUser Alice Bob
+```
+6. Attempt to reveal the secret as Bob (should fail):
+```
+./fpcclient query revealSecret Bob // (will failed) 
+```
+7. Re-add Bob to the authorization list as Alice:
+```
+./fpcclient invoke addUser Alice Bob
+```
+8. Bob can now reveal the secret successfully:
+```
+./fpcclient query revealSecret Bob // (will success)
+```