Document stage: N labels

These are described in #290.

LABELS.md

@@ -30,6 +30,11 @@ These are labels used by all [WHATWG standards](https://spec.whatwg.org/):
 * [needs tests](https://github.com/search?q=org%3Awhatwg+label%3A%22needs+tests%22+is%3Aopen): Moving the issue forward requires someone to write tests
 * [removal/deprecation](https://github.com/search?q=org%3Awhatwg+label%3A%22removal%2Fdeprecation%22+is%3Aopen): Removing or deprecating a feature
 * [security/privacy](https://github.com/search?q=org%3Awhatwg+label%3A%22security%2Fprivacy%22+is%3Aopen): There are security or privacy implications
+* [stage: 0](https://github.com/search?q=org%3Awhatwg+label%3A%22stage%3A+0%22+is%3Aopen): Proposal
+* [stage: 1](https://github.com/search?q=org%3Awhatwg+label%3A%22stage%3A+1%22+is%3Aopen): Incubation
+* [stage: 2](https://github.com/search?q=org%3Awhatwg+label%3A%22stage%3A+2%22+is%3Aopen): Iteration
+* [stage: 3](https://github.com/search?q=org%3Awhatwg+label%3A%22stage%3A+3%22+is%3Aopen): Committed
+* [stage: 4](https://github.com/search?q=org%3Awhatwg+label%3A%22stage%3A+4%22+is%3Aopen): Standard
 * [topic: custom elements](https://github.com/search?q=org%3Awhatwg+label%3A%22topic%3A+custom+elements%22+is%3Aopen): Relates to custom elements (as defined in DOM and HTML)
 * [topic: shadow](https://github.com/search?q=org%3Awhatwg+label%3A%22topic%3A+shadow%22+is%3Aopen): Relates to shadow trees (as defined in DOM)
 

labels.json

@@ -179,6 +179,31 @@
     "description": "There are security or privacy implications",
     "name": "security/privacy"
   },
+  {
+    "color": "1d76db",
+    "description": "Proposal",
+    "name": "stage: 0"
+  },
+  {
+    "color": "1d76db",
+    "description": "Incubation",
+    "name": "stage: 1"
+  },
+  {
+    "color": "1d76db",
+    "description": "Iteration",
+    "name": "stage: 2"
+  },
+  {
+    "color": "1d76db",
+    "description": "Committed",
+    "name": "stage: 3"
+  },
+  {
+    "color": "1d76db",
+    "description": "Standard",
+    "name": "stage: 4"
+  },
   {
     "color": "7fff00",
     "description": "Issue the Technical Architecture Group has raised and looks for a response on.",