docs: add note about literal alias limitation (#401)

Track issue in viper: spf13/viper#1014

Co-authored-by: Ember Stevens <[email protected]>

Author: jazanne

docs/ALIASES.md

@@ -35,6 +35,8 @@ var secondFeatureFlag = 'second-flag-key'
 
 Aliases can be hardcoded using the `literal` type. This is intended to be used for testing aliasing functionality.
 
+Note that literal aliases are limited to flag keys using lowercase letters only, due to a limitation of a dependency LaunchDarkly uses to parse the configuration file. [You can track the issue here](https://github.com/spf13/viper/issues/1014).
+
 Example hardcoding aliases for a couple flags:
 
 ```yaml