add exports and library info to README.md

src/grammar.json

@@ -1313,40 +1313,44 @@
           ]
         },
         {
-          "type": "SEQ",
+          "type": "SYMBOL",
+          "name": "_custom_filter"
+        }
+      ]
+    },
+    "_custom_filter": {
+      "type": "SEQ",
+      "members": [
+        {
+          "type": "FIELD",
+          "name": "name",
+          "content": {
+            "type": "SYMBOL",
+            "name": "identifier"
+          }
+        },
+        {
+          "type": "CHOICE",
           "members": [
             {
-              "type": "FIELD",
-              "name": "name",
-              "content": {
-                "type": "SYMBOL",
-                "name": "identifier"
-              }
-            },
-            {
-              "type": "CHOICE",
+              "type": "SEQ",
               "members": [
                 {
-                  "type": "SEQ",
-                  "members": [
-                    {
-                      "type": "STRING",
-                      "value": ":"
-                    },
-                    {
-                      "type": "FIELD",
-                      "name": "argument",
-                      "content": {
-                        "type": "SYMBOL",
-                        "name": "value"
-                      }
-                    }
-                  ]
+                  "type": "STRING",
+                  "value": ":"
                 },
                 {
-                  "type": "BLANK"
+                  "type": "FIELD",
+                  "name": "argument",
+                  "content": {
+                    "type": "SYMBOL",
+                    "name": "value"
+                  }
                 }
               ]
+            },
+            {
+              "type": "BLANK"
             }
           ]
         }