Update pkg readme (#510)

Author: bitfaster

BitFaster.Caching/ReadMe.md

@@ -15,6 +15,11 @@ var lru = new ConcurrentLru<string, SomeItem>(capacity);
 var value = lru.GetOrAdd("key", (key) => new SomeItem(key));
 ```
 
+Optionally configure `ConcurrentLru` with a [time-based eviction policy](https://github.com/bitfaster/BitFaster.Caching/wiki/ConcurrentLru:-Time%E2%80%90based-eviction), either:
+- Expire after write
+- Expire after access
+- Calculate an expiry time per item
+
 ## ConcurrentLfu
 
 `ConcurrentLfu` is a drop in replacement for `ConcurrentDictionary`, but with bounded size enforced by the [W-TinyLFU admission policy](https://arxiv.org/pdf/1512.00727.pdf). `ConcurrentLfu` has near optimal hit rate and high scalability. Reads and writes are buffered then replayed asynchronously to mitigate lock contention.