Merge pull request #597 from kubukoz/patch-1

Add a note about classloader issues to README

README.md

@@ -759,6 +759,15 @@ If you have trouble with your configuration, some useful tips.
    string with comments showing where each value came from.
    This string can be printed out on console or logged to 
    a file etc.
+ - If you see errors like
+   `com.typesafe.config.ConfigException$Missing: No configuration setting found for key foo`,
+   and you're sure that key is defined in your config file, they might appear e.g.
+   when you're loading configuration from a thread that's not the JVM's main thread.
+   Try passing the `ClassLoader` in manually - e.g. with `ConfigFactory.load(getClass().getClassLoader())`
+   or setting the context class loader.
+   If you don't pass one, Lightbend Config uses the calling thread's `contextClassLoader`, and in some cases,
+   it may not have your configuration files in its classpath,
+   so loading the config on that thread can yield unexpected, erroneous results.
 
 ### Supports Java 8 and Later