Add a known issue for debugger (#148)

Author: axelson

README.md

@@ -120,6 +120,7 @@ If your code doesn't compile in ElixirLS, it may be because ElixirLS compiles co
 * `.exs` files don't return compilation errors
 * `workspaceSymbolProvider` capability currently requires enabled dialyzer
 * "Fetching n dependencies" sometimes get stuck (remove the `.elixir_ls` directory to fix)
+* Debugger doesn't work in Elixir 1.10.0 - 1.10.2 (but it should work in 1.10.3 when [this fix](https://github.com/elixir-lang/elixir/pull/9864) is released)
 
 ## Building and running