Docs: show "unreleased" warning on latest version

Fixes #52

docs/_static/version-alert.js

@@ -0,0 +1,30 @@
+"use strict";
+
+// Similar to https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/core/static-src/core/js/doc-embed/version-compare.js
+// but adds admonition for the "latest" version -- which is (unreleased) master branch.
+
+function warnOnLatestVersion() {
+
+  // The warning text and link is really specific to RTD hosting,
+  // so we can just check their global to determine version:
+  if (!window.READTHEDOCS_DATA || window.READTHEDOCS_DATA.version !== "latest") {
+    return;  // not latest, or not on RTD
+  }
+
+  var warning = document.createElement('div');
+  warning.setAttribute('class', 'admonition danger');
+  warning.innerHTML = "<p class='first admonition-title'>Note</p> " +
+    "<p class='last'> " +
+    "This document is for an <strong>unreleased development version</strong>. " +
+    "Documentation is available for the <a href='/en/stable/'>current stable release</a>, " +
+    "or for older versions through the &ldquo;v:&rdquo; menu at bottom left." +
+    "</p>";
+  warning.querySelector('a').href = window.location.pathname.replace('/latest', '/stable');
+
+  var parent = document.querySelector('div.body')
+    || document.querySelector('div.document')
+    || document.body;
+  parent.insertBefore(warning, parent.firstChild);
+}
+
+document.addEventListener('DOMContentLoaded', warnOnLatestVersion);

docs/_templates/layout.html

@@ -0,0 +1,3 @@
+{% extends "!layout.html" %}
+
+{% set script_files = script_files + ["_static/version-alert.js"] %}