Created known issues page

Known-Issues.md

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+# Known issues with EssentialsX
+
+## Signs don't work
+In your `plugins/Essentials/config.yml` file, you need to uncomment the lines under `enabledSigns` for each type of sign by removing the `#` before it:
+```yaml
+enabledSigns:
+  #- color
+  - balance
+  #- buy
+```
+In this example, the `[balance]` sign is enabled but the `[buy]` sign is not.  
+Note that enabling `color` means that players will be allowed to use color codes in the sign text.
+
+## EssentialsX overrides a command from Spigot or another plugin
+**Related issues: [#1458](/drtshock/Essentials/issue/1458)**  
+You can create an alias for commands using Bukkit's `commands.yml` file, which should be in your server root. For example:
+```yaml
+aliases:
+    ban:
+    - "<plugin>:ban $1-"
+    tell:
+    - "minecraft:tell $1-"
+```
+where `<plugin>` is a placeholder for a plugin name.
+
+See the [Bukkit wiki page](https://bukkit.gamepedia.com/Commands.yml#aliases) for more information.
+
+## Tab completion doesn't work for commands that override an EssentialsX command
+**Related issues: [#1384](/drtshock/Essentials/issue/1384)**  
+You can alias the command to the version from the other plugin, which should fix tab complete behaviour - see above.