docs: update assertions docs for new functionality

Author: patrickhulce

docs/assertions.md

@@ -31,6 +31,19 @@ The result of any audit in Lighthouse can be asserted. Assertions are keyed by t
 }
 ```
 
+### Categories
+
+The score of any category in Lighthouse can also be asserted. Assertions are keyed by `categories.<categoryId>` and follow the same eslint-style format as audit assertions.
+
+```json
+{
+  "assertions": {
+    "categories.pwa": "off",
+    "categories.accessibility": ["error", {"this-is-an-options-object": true}]
+  }
+}
+```
+
 ### Levels
 
 There are three Lighthouse CI assertion levels.
@@ -114,3 +127,17 @@ Instead of configuring using Lighthouse CI assertions against Lighthouse audits,
   "budgetsFile": "path/from/cwd/to/budget.json"
 }
 ```
+
+If you'd like to consolidate multiple assertion configuration files and avoid multiple calls to `lhci assert`, you can also configure your budgets alongside your other Lighthouse CI assertions instead. Budget assertions follow the form `resource-summary.<resourceType>.(size|count)`.
+
+```json
+{
+  "assertions": {
+    "audit-id-1": ["warn", {"maxNumericValue": 4000}],
+    "audit-id-2": ["error", {"minScore": 0.8}],
+    "resource-summary.document.size": ["error", {"maxNumericValue": 400}],
+    "resource-summary.font.count": ["warn", {"maxNumericValue": 1}],
+    "resource-summary.third-party.count": ["warn", {"maxNumericValue": 5}]
+  }
+}
+```