Added the `has` method, enhanced README and documentation

Key Features:

• Added the has method: Determines whether the cache contains a valid, non-expired entry for the specified key. This method is particularly useful when the cache is employed as a Set-like structure, where the presence of a key is significant but the associated value is secondary or unnecessary.
• README enhancements: Added an additional example highlighting scenarios where a FIFO cache is preferable to an LRU cache. Documented the newly added has method, and updated terminology from "cached records" to "cached entries".
• Documentation enhancements: Improved clarity by addressing subtle nuances and refining explanations.