Add align content documentation (#1605)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1605

tsia

Reviewed By: yungsters

Differential Revision: D54782553

fbshipit-source-id: a65aaa754ea98a8dfe3b581a7cdb06dddf35f64a

website-next/docs/examples/align-content.mdx

@@ -6,10 +6,37 @@ import Playground from '@site/src/components/Playground';
 
 # Align Content
 
+Align content defines the distribution of lines along the cross-axis. This only
+has effect when items are wrapped to multiple lines using [`flex wrap`](/docs/examples/flex-wrap).
+
+**Flex start (default)**: Align wrapped lines to the start of the container's cross axis.
+
+**Flex end**: Align wrapped lines to the end of the container's cross axis.
+
+**Stretch**: Stretch wrapped lines to match the `height` of the container's cross axis.
+
+**Center**: Align wrapped lines in the center of the container's cross axis.
+
+**Space between**: Evenly space wrapped lines across the container's main axis, distributing
+remaining space between the lines.
+
+**Space around**: Evenly space wrapped lines across the container's main axis, distributing
+remaining space around the lines. Compared to `space between` using
+`space around` will result in space being distributed to the begining of
+the first lines and end of the last line.
+
 <Playground code={`<Layout config={{useWebDefaults: false}}>
   <Node
-    style={{width: 200, height: 200, padding: 20}}
-  >
-    <Node style={{flex: 1}} />
+    style={{
+      width: 250,
+      height: 250,
+      padding: 10,
+      alignContent: 'flex-start',
+      flexWrap: 'wrap'
+    }}>
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
   </Node>
 </Layout>`} />