docs(README): Add note about flex bug.

Author: jayfreestone

README.md

@@ -239,3 +239,36 @@ Simply add a rule that causes the first item in the navigation to expand beyond
   }
 }
 ```
+
+## Troubleshooting
+
+## Flex nav collapsing
+
+If your menu is part of an auto-sized flex-child, it will probably need a positive `flex-grow` value to prevent it reverting to its smallest form. For instance:
+
+```html
+<header class="site-header">
+  <h1 class="site-header__title">My great site title</h1>
+  <nav class="site-header__nav">
+		<ul class="site-nav js-site-nav">
+		  <li><a href="#">Services</a></li>
+		  <li><a href="#">Thinking</a></li>
+		  <li><a href="#">Events</a></li>
+		</ul>
+  </nav>
+</header>
+```
+
+```css
+.site-header {
+	display: flex;
+	align-items: center;
+}
+
+/**
+ * Prevents nav from collapsing.
+ */
+.site-header__nav {
+	flex-grow: 1;
+}
+```