eslint-plugin: add docs for no-forbidden-package-imports

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/eslint-plugin/docs/rules/no-forbidden-package-imports.md

@@ -0,0 +1,52 @@
+# @backstage/no-forbidden-package-imports
+
+Disallow internal monorepo imports from package subpaths that are not exported.
+
+## Usage
+
+Add the rules as follows, it has no options:
+
+```js
+"@backstage/no-forbidden-package-imports": ["error"]
+```
+
+## Rule Details
+
+Given the following two target packages:
+
+```json
+{
+  "name": "@backstage/plugin-foo",
+  "files": ["dist", "type-utils"]
+}
+```
+
+```json
+{
+  "name": "@backstage/plugin-bar",
+  "exports": {
+    ".": "./src/index.ts",
+    "./testUtils": "./src/testUtils/index.ts",
+    "./package.json": "./package.json"
+  }
+}
+```
+
+### Fail
+
+```ts
+import { FooCard } from '@backstage/plugin-foo/src/components';
+import { BarCard } from '@backstage/plugin-bar/src/components';
+```
+
+### Pass
+
+```ts
+import { FooCard } from '@backstage/plugin-foo';
+import { FooType } from '@backstage/plugin-foo/type-utils';
+import fooPkg from '@backstage/plugin-foo/package.json';
+
+import { BarCard } from '@backstage/plugin-bar';
+import { renderBarCardExtension } from '@backstage/plugin-bar/testUtils';
+import barPkg from '@backstage/plugin-bar/package.json';
+```