Auto merge of rust-lang#2185 - rye:document-categories, r=jtgeibel

frontend: Document category proposal system and categorization process

This PR aims to close rust-lang#544, and just adds a div and some styles for it.

<details><summary>📸 Before</summary><img width="1036" alt="image" src="https://user-images.githubusercontent.com/1566689/74396257-0aa34100-4dd7-11ea-9f77-c27cca629da6.png"></details>
<details><summary>📸 After</summary><img width="1040" alt="image" src="https://user-images.githubusercontent.com/1566689/74396288-260e4c00-4dd7-11ea-9c48-04bf3f9c814b.png"></details>

Wording and stylesheet placement is absolutely negotiable!

app/styles/app.scss

@@ -13,6 +13,7 @@ $link-color: rgb(0, 172, 91);
 @import "application";
 @import "home";
 @import "category-slugs";
+@import "categories";
 @import "crate";
 @import "crate/version";
 @import "components/crate-toml-copy";

app/styles/categories.scss

@@ -0,0 +1,8 @@
+/* Styles for the /categories page */
+
+.categories-footer {
+	width: 100%;
+	padding: 1em;
+	text-align: center;
+	font-size: 85%;
+}

app/templates/categories.hbs

@@ -60,3 +60,8 @@
 </div>
 
 <Pagination @pages={{this.pages}} @prevPage={{this.prevPage}} @nextPage={{this.nextPage}} />
+
+<div class='categories-footer'>
+  Want to categorize your crate?
+  <a href='http://doc.crates.io/manifest.html#package-metadata'>Add metadata!</a>
+</div>