Add a "this is unpublished header" to docs

.travis.yml

@@ -10,9 +10,9 @@ before_script:
   - cargo install mdbook --version 0.2.3
 
 script:
+  - mv _theme theme
   - mdbook build
 
-
 deploy:
   provider: script
   script: curl -LsSf https://git.io/fhJ8n | rustc - && (cd book && ../rust_out)

_theme/header.hbs

@@ -0,0 +1,44 @@
+<style>
+  header.warning {
+    background-color: rgb(242, 222, 222);
+    border-bottom-color: rgb(238, 211, 215);
+    border-bottom-left-radius: 4px;
+    border-bottom-right-radius: 4px;
+    border-bottom-style: solid;
+    border-bottom-width: 0.666667px;
+    border-image-outset: 0 0 0 0;
+    border-image-repeat: stretch stretch;
+    border-image-slice: 100% 100% 100% 100%;
+    border-image-source: none;
+    border-image-width: 1 1 1 1;
+    border-left-color: rgb(238, 211, 215);
+    border-left-style: solid;
+    border-left-width: 0.666667px;
+    border-right-color: rgb(238, 211, 215);
+    border-right-style: solid;
+    border-right-width: 0.666667px;
+    border-top-color: rgb(238, 211, 215);
+    border-top-left-radius: 4px;
+    border-top-right-radius: 4px;
+    border-top-style: solid;
+    border-top-width: 0.666667px;
+    color: rgb(185, 74, 72);
+    margin-bottom: 0px;
+    margin-left: 0px;
+    margin-right: 0px;
+    margin-top: 30px;
+    padding-bottom: 8px;
+    padding-left: 14px;
+    padding-right: 35px;
+    padding-top: 8px;
+    text-align: center;
+  }
+</style>
+<header class='warning'>
+  This is <strong>unpublished</strong> documentation of
+  working with Rust and WebAssembly, the published documentation is available
+  <a href="https://rustwasm.github.io/docs/book/">
+    on the main Rust and WebAssembly documentation site
+  </a>. Features documented here may not be available in released versions of
+  tooling for Rust and WebAssembly.
+</header>