updated readme with disclaimer on Enum values

[DjoykeAbyah]

Description

This PR updates the README to include a disclaimer regarding the potential for enum values to change in future releases. This change ensures that users are aware that they should not rely on the enum values being static, and should handle them dynamically or with caution in their implementations.

• Added a new section in the README under ## Disclaimer on Enum Values.
• Explained that enum values may be modified, added, or removed in future releases.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[Kwok-he-Chu]

I've found several cases (but not limited to these* only), this will become a problem if the underlying integer value is stored instead of the respective string-value.

• https://github.com/Adyen/adyen-dotnet-api-library/pull/1037/files#diff-654e173618021475129caec200e9fd730e8f1ae2b5ee59966779fec564b28835L64
• https://github.com/Adyen/adyen-dotnet-api-library/pull/1031/files#diff-923be669a9c28428736a92f8f172cb98a48a765df4992204f74b4ecd4443fb2aR226
• https://github.com/Adyen/adyen-dotnet-api-library/pull/1026/files#diff-44bbf43931d61abce260295e53f7988d0b32d407dde99e3c877b39ec56a3036bR192

I have opened an issue/discussion for now. As of currently, adding one (alphabetical-ordered) enum in the OpenApi specifications results in an immediate breaking change when generating the library