add ASP.NET know issues for .NET 4.8 release

Documentation/KnownIssues/776516-InvalidOperationException in System_Web_ni!System.Web.Hosting.RecycleLimitMonitor+RecycleLimitMonitorSingleton.AlertProxyMonitors.md

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+# InvalidOperationException in ASP.Net-RecycleLimitMonitor-AlertProxyMonitors
+
+## Symptoms
+ASP.Net applications may crash intermittently due to an `InvalidOperationException` in
+`System_Web_ni!System.Web.Hosting.RecycleLimitMonitor+RecycleLimitMonitorSingleton.AlertProxyMonitors()`.
+
+## Cause
+This issue was introduced when ASP.Net 4.7 separated the concerns of cache management and memory monitoring. This new
+`RecycleLimitMonitorSingleton` monitors worker process private bytes usage against the recycling limit for the process, and when
+the limit is near, it alerts a set of "proxy monitors" in each registered AppDomain so they can take action to avoid a process
+recycle. The collection of "proxy monitors" being enumerated was not thread-safe though, so new AppDomains spinning up or old
+AppDomains shutting down while this monitor enumerated the collection could result in this unhandled `InvalidOperationException`
+in the default AppDomain, causing a crash.
+
+## Workaround 
+There is no reliable workaround. If the environment does not tend to consume private bytes near the recycling limit, or if
+web app domain lifetimes tend to last the entire length of the process lifetime, then chances of hitting this issue are
+extremely minimal. More private bytes consumption and/or more AppDomain recycling will increase the odds of hitting this.
+
+## Resolution
+A hotfix for this issue is in the works for .Net 4.8. This page will be updated with a link to the hotfix when it becomes
+available.
+
+## More Information
+