fix: 🐛 Error with flow opaque type and readme command

When exporting an opaque Flow type and importing it into an entry file
for documentation to consume, the CLI throws an error when using the
readme command.

✅ Closes: #1322

Author: andrewdelprete

__tests__/__snapshots__/test.js.snap

@@ -18930,6 +18930,185 @@ Object {
 }
 `;
 
+exports[`outputs flow-exported-opaque-type.input.js JSON 1`] = `
+Array [
+  Object {
+    "augments": Array [],
+    "context": Object {
+      "loc": Object {
+        "end": Object {
+          "column": 48,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 0,
+          "line": 4,
+        },
+      },
+    },
+    "description": Object {
+      "children": Array [
+        Object {
+          "children": Array [
+            Object {
+              "position": Object {
+                "end": Object {
+                  "column": 30,
+                  "line": 1,
+                  "offset": 29,
+                },
+                "indent": Array [],
+                "start": Object {
+                  "column": 1,
+                  "line": 1,
+                  "offset": 0,
+                },
+              },
+              "type": "text",
+              "value": "MyOpaqueType Description Here",
+            },
+          ],
+          "position": Object {
+            "end": Object {
+              "column": 30,
+              "line": 1,
+              "offset": 29,
+            },
+            "indent": Array [],
+            "start": Object {
+              "column": 1,
+              "line": 1,
+              "offset": 0,
+            },
+          },
+          "type": "paragraph",
+        },
+      ],
+      "position": Object {
+        "end": Object {
+          "column": 30,
+          "line": 1,
+          "offset": 29,
+        },
+        "start": Object {
+          "column": 1,
+          "line": 1,
+          "offset": 0,
+        },
+      },
+      "type": "root",
+    },
+    "errors": Array [],
+    "examples": Array [],
+    "implements": Array [],
+    "loc": Object {
+      "end": Object {
+        "column": 36,
+        "line": 3,
+      },
+      "start": Object {
+        "column": 0,
+        "line": 3,
+      },
+    },
+    "members": Object {
+      "events": Array [],
+      "global": Array [],
+      "inner": Array [],
+      "instance": Array [],
+      "static": Array [],
+    },
+    "name": "MyOpaqueType",
+    "namespace": "MyOpaqueType",
+    "params": Array [],
+    "path": Array [
+      Object {
+        "name": "MyOpaqueType",
+      },
+    ],
+    "properties": Array [],
+    "returns": Array [],
+    "sees": Array [],
+    "tags": Array [],
+    "throws": Array [],
+    "todos": Array [],
+    "yields": Array [],
+  },
+]
+`;
+
+exports[`outputs flow-exported-opaque-type.input.js markdown 1`] = `
+"<!-- Generated by documentation.js. Update this documentation by updating the source code. -->
+
+### Table of Contents
+
+-   [MyOpaqueType][1]
+
+## MyOpaqueType
+
+MyOpaqueType Description Here
+
+[1]: #myopaquetype
+"
+`;
+
+exports[`outputs flow-exported-opaque-type.input.js markdown AST 1`] = `
+Object {
+  "children": Array [
+    Object {
+      "type": "html",
+      "value": "<!-- Generated by documentation.js. Update this documentation by updating the source code. -->",
+    },
+    Object {
+      "children": Array [
+        Object {
+          "type": "text",
+          "value": "MyOpaqueType",
+        },
+      ],
+      "depth": 2,
+      "type": "heading",
+    },
+    Object {
+      "children": Array [
+        Object {
+          "position": Position {
+            "end": Object {
+              "column": 30,
+              "line": 1,
+              "offset": 29,
+            },
+            "indent": Array [],
+            "start": Object {
+              "column": 1,
+              "line": 1,
+              "offset": 0,
+            },
+          },
+          "type": "text",
+          "value": "MyOpaqueType Description Here",
+        },
+      ],
+      "position": Position {
+        "end": Object {
+          "column": 30,
+          "line": 1,
+          "offset": 29,
+        },
+        "indent": Array [],
+        "start": Object {
+          "column": 1,
+          "line": 1,
+          "offset": 0,
+        },
+      },
+      "type": "paragraph",
+    },
+  ],
+  "type": "root",
+}
+`;
+
 exports[`outputs flow-optional-chaining.input.js JSON 1`] = `
 Array [
   Object {

__tests__/fixture/flow-exported-opaque-type.input.js

@@ -0,0 +1,4 @@
+// @flow
+
+/** MyOpaqueType Description Here */
+export opaque type MyOpaqueType: string = string

src/extractors/exported.js

@@ -224,7 +224,8 @@ function findExportDeclaration(
           if (
             declaration.isFunctionDeclaration() ||
             declaration.isClassDeclaration() ||
-            declaration.isTypeAlias()
+            declaration.isTypeAlias() ||
+            declaration.isOpaqueType()
           ) {
             bindingName = declaration.node.id.name;
           } else if (declaration.isVariableDeclaration()) {