Clarify username field is not for logging in

src/components/registration/steps.jsx

@@ -102,7 +102,7 @@ module.exports = {
                     </p>
                     <Card>
                         <Form onValidSubmit={this.onValidSubmit}>
-                            <Input label={formatMessage({id: 'general.username'})}
+                            <Input label={formatMessage({id: 'general.createUsername'})}
                                    className={this.state.validUsername}
                                    type="text"
                                    name="user.username"

src/l10n.json

@@ -12,6 +12,7 @@
     "general.copyright": "Scratch is a project of the Lifelong Kindergarten Group at the MIT Media Lab",
     "general.country": "Country",
     "general.create": "Create",
+    "general.createUsername": "Create a Username",
     "general.credits": "Credits",
     "general.discuss": "Discuss",
     "general.dmca": "DMCA",