Add an incomplete warning to the top page

Author: JohnTitor

Diff for: book.toml

@@ -4,4 +4,5 @@ title = "The Rustonomicon"
 description = "The Dark Arts of Advanced and Unsafe Rust Programming"
 
 [output.html]
+additional-css = ["theme/nomicon.css"]
 git-repository-url = "https://github.com/rust-lang/nomicon"

Diff for: src/README.md

@@ -1,5 +1,16 @@
 # The Rustonomicon
 
+<div class="warning">
+
+Warning:
+This book is incomplete. Documenting everything and rewriting outdated parts take a while.
+See the [issue tracker] to check what's missing/outdated, and if there are any mistakes
+or ideas that haven't been reported, feel free to open a new issue there.
+
+</div>
+
+[issue tracker]: https://github.com/rust-lang/nomicon/issues
+
 #### The Dark Arts of Unsafe Rust
 
 > THE KNOWLEDGE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,

Diff for: theme/nomicon.css

@@ -0,0 +1,40 @@
+/*
+Taken from the reference.
+Warnings and notes:
+Write the <div>s on their own line. E.g.
+<div class="warning">
+Warning: This is bad!
+</div>
+*/
+main .warning p {
+    padding: 10px 20px;
+    margin: 20px 0;
+}
+
+main .warning p::before {
+    content: "⚠️ ";
+}
+
+.light main .warning p,
+.rust main .warning p {
+    border: 2px solid red;
+    background: #ffcece;
+}
+
+.rust main .warning p {
+    /* overrides previous declaration */
+    border-color: #961717;
+}
+
+.coal main .warning p,
+.navy main .warning p,
+.ayu main .warning p {
+    background: #542626
+}
+
+/* Make the links higher contrast on dark themes */
+.coal main .warning p a,
+.navy main .warning p a,
+.ayu main .warning p a {
+    color: #80d0d0
+}