Raise explicit setup issues about PHP 7

Summary: Ref T9640. Fixes T9888. Decline to support PHP 7 until the async signal handling issue in T11270 is resolved.

Test Plan: Faked local version, got helpful error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640, T9888

Differential Revision: https://secure.phabricator.com/D16231

src/docs/user/installation_guide.diviner

@@ -70,7 +70,8 @@ Beyond an operating system, you will need **a webserver**.
 You will also need:
 
   - **MySQL**: You need MySQL. We strongly recommend MySQL 5.5 or newer.
-  - **PHP**: You need PHP 5.2 or newer.
+  - **PHP**: You need PHP 5.2 or newer, but note that PHP 7 is
+    **not supported**.
 
 You'll probably also need a **domain name**. In particular, you should read this
 note: