Make landing message a bit more positive and concise.

templates/zerver/hello.html

@@ -32,9 +32,9 @@
                 <h1>The world's most productive group chat</h1>
                 <p>
                     Zulip combines the immediacy of Slack with an email
-                    threading model.  <br />With Zulip, you don't have to
-                    choose between missing important conversations and
-                    wasting time reading irrelevant ones.
+                    threading model.  <br />With Zulip, you can catch
+                    up on important conversations while ignoring
+                    irrelevant ones.
                 </p>
                 {% if register_link_disabled %}
                 {% elif only_sso %}