Add a helpful link to the cred explorer (#727)

Adds a link titled "what is this?" that points to my gentle introduction
to cred. Also, move the feedback link to be next to it and get rid of
the prototype disclaimer.

Test plan: Visual inspection, also a test was updated.

src/app/credExplorer/App.js

@@ -80,15 +80,21 @@ export function createApp(
           />
         );
       }
+      const spacer = () => (
+        <span style={{display: "inline-block", width: 12}} />
+      );
       return (
         <div style={{maxWidth: 900, margin: "0 auto", padding: "0 10px"}}>
-          <p>
-            <em>
-              This is a prototype. Please{" "}
-              <a href={process.env.SOURCECRED_FEEDBACK_URL}>
-                let us know what you think
-              </a>.
-            </em>
+          <p style={{textAlign: "right"}}>
+            <a
+              href={
+                "https://discuss.sourcecred.io/t/a-gentle-introduction-to-cred/20"
+              }
+            >
+              what is this?
+            </a>
+            {spacer()}
+            <a href={process.env.SOURCECRED_FEEDBACK_URL}>feedback</a>
           </p>
           <div style={{marginBottom: 10}}>
             <RepositorySelect

src/app/credExplorer/App.test.js

@@ -116,9 +116,7 @@ describe("app/credExplorer/App", () => {
 
   it("should have a feedback link with a valid URL", () => {
     const {el} = example();
-    const link = el
-      .find("a")
-      .filterWhere((x) => x.text().includes("let us know what you think"));
+    const link = el.find("a").filterWhere((x) => x.text().includes("feedback"));
     expect(link).toHaveLength(1);
     expect(link.prop("href")).toMatch(/https?:\/\//);
   });