Add known issues documentation for vpc-go-sdk (#147)

* Add known issues documentation for vpc-go-sdk

Document known issues related to security group and network ACL rules with patch fix versions.

* Update known-issues.md

Signed-off-by: Astha Jain <astha.jain@ibm.com>

* update as per review

Signed-off-by: Astha Jain <astha.jain@ibm.com>

---------

Signed-off-by: Astha Jain <astha.jain@ibm.com>

Author: astha-jain

known-issues.md

@@ -0,0 +1,34 @@
+# Known Issues for vpc-go-sdk
+
+## Security Group Rules and Network ACL Rules - discriminator issue
+#### Publication date - "2025-12-18"
+
+### Patch fix versions available in the following versions - 
+[v0.76.5](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.76.5)
+[v0.75.1](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.75.1)
+[v0.74.2](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.74.2)
+[v0.73.1](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.73.1)
+[v0.72.1](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.72.1)
+[v0.71.2](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.71.2)
+[v0.70.2](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.70.2)
+[v0.69.2](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.69.2)
+[v0.68.1](https://github.com/IBM/vpc-go-sdk/releases/tag/v0.68.1)
+and later versions
+
+### Affected component
+vpc-go-sdk
+
+### Affected operations
+Security group rules and Network ACL rules 
+
+### Issue Summary
+#### Support for `any` Protocol in Security Group Rules and Network ACL Rules
+Security Group rules and Network ACL rules now support the new protocol value `any`. Earlier versions of SDK could result in following error reading a rule with an unsupported protocol value. 
+```
+error unmarshalling vpcv1.SecurityGroupCollection: error unmarshalling property 'security_groups' as []vpcv1.SecurityGroup: error unmarshalling property 'rules' as []vpcv1.SecurityGroupRuleIntf: unrecognized value for discriminator property 'protocol': any
+```
+
+Fix aligns fallback behavior and error identifiers with the correct model name ( e.g. NetworkACLRule instead of NetworkACLRuleItem).
+
+### Migration & mitigation
+Migrate to one of the patched versions, or update the SDK to the [latest version](https://github.com/IBM/vpc-go-sdk/releases)