Add a notice for the cache duration precision issue. (#119)

see #118

Author: Stumble

README.md

@@ -56,6 +56,11 @@ fmt.Println("entry count ", cache.EntryCount())
 * Memory is preallocated.
 * If you allocate large amount of memory, you may need to set `debug.SetGCPercent()`
 to a much lower percentage to get a normal GC frequency.
+* If you set a key to be expired in X seconds, e.g. using `cache.Set(key, val, X)`, 
+the effective cache duration will be within this range: `(X-1, X] seconds`.
+This is because that sub-second time at the moment will be ignored when calculating the
+the expiration: for example, if the current time is 8:15::01.800 (800 milliseconds passed
+since 8:15::01), the actual duration will be `X-800ms`.
 
 ## How it is done