Add missing information about tsconfig.json

That a file tsconfig.json is necessary was not mentioned (without it the error "The 'files' list in config file 'tsconfig.json' is empty" occurs), and making it clear that it does not have to be in the main project folder seems helpful.

Author: iquito

frontend/encore/typescript.rst

@@ -20,6 +20,11 @@ Want to use `TypeScript`_? No problem! First, enable it:
     +     //.enableForkedTypeScriptTypesChecking()
       ;
 
+Then add a `tsconfig.json` file with the contents `{}` to the project folder,
+or in the folder where your TypeScript files are located, like the `/assets`
+folder. In `tsconfig.json` you can define more options on how to process
+TypeScript, but defining no additional options will already work.
+
 Then restart Encore. When you do, it will give you a command you can run to
 install any missing dependencies. After running that command and restarting
 Encore, you're done!