Updated Known Issues (markdown)

Known-Issues.md

@@ -1,6 +1,7 @@
 This page lists some of the issues one can encounter when using polybar. Either quirks in polybar's behaviour or actual bugs we haven't been able to fix yet.
 
 # Contents
+- [Click Commands are not Executed](#click-commands-are-not-executed)
 - [With multiple modules of the same type, actions on those modules always control the leftmost module](#with-multiple-modules-of-the-same-type-actions-on-those-modules-control-the-leftmost-module)
 - [Error While Loading Shared Libraries: libjsoncpp](#error-while-loading-shared-libraries-libjsoncpp)
 - [Setting Module Backgrounds when using Gradients](#setting-module-backgrounds-when-using-gradients)
@@ -8,6 +9,13 @@ This page lists some of the issues one can encounter when using polybar. Either
 - [KeyError: 'eventstruct'](#keyerror-eventstruct)
 - [Version Mismatch between `xcb-proto` and `libxcb-randr0-dev` on Ubuntu 17.04](#version-mismatch-between-xcb-proto-and-libxcb-randr0-dev-on-ubuntu-1704)
 
+## Click Commands are not Executed
+**Problem:** Some `click-*` commands are not executed when the bar is clicked. This is a bug that is triggered by the `click-*` command producing output on `stdout` or `stderr`.
+
+**Workaround:** The `click-*` command must not produce any output, so redirect any output to `/dev/null`. This can be done by appending `>/dev/null 2>&1` to the `click-*` command.
+
+This issue was discovered in [`#916`](https://github.com/jaagr/polybar/issues/916).
+
 ## With multiple modules of the same type, actions on those modules control the leftmost module
 **Problem:** This affects all modules that provide their internal mouse actions that change the module behaviour:
 * `internal/alsa`