Add project-specific release notes for 1.5.9

Author: odinserj

nuspecs/Hangfire.Core.nuspec

@@ -14,6 +14,9 @@
     <tags>Hangfire OWIN Long-Running Background Fire-And-Forget Delayed Recurring Tasks Jobs Scheduler Threading Queues</tags>
     <releaseNotes>http://hangfire.io/blog/
 
+1.5.9
+* Fixed – `OperationCanceledException` exceptions don't cause background processing to stop.
+    
 1.5.7
 * Fixed – Add queue name validation to the `RecurringJobScheduler` class (by @pzielinski86).
 

nuspecs/Hangfire.SqlServer.nuspec

@@ -14,6 +14,10 @@
     <tags>Hangfire SqlServer SqlAzure LocalDB</tags>
     <releaseNotes>http://hangfire.io/blog/
 
+1.5.9
+* Fixed – Don't leak Dashboard UI's `READ UNCOMMITTED` isolation level to other parts of an application.
+* Fixed – `READ_COMMITTED_SNAPSHOT` option doesn't prevent background jobs from being processed.
+    
 1.5.8
 * Changed – Disable immediate execution of a keep-alive query in queue and distributed locks.
 * Fixed – Background jobs may be processed multiple times.