eslint-plugin: add docs for no-relative-monorepo-imports

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

packages/eslint-plugin/docs/rules/no-relative-monorepo-imports.md

@@ -0,0 +1,43 @@
+# @backstage/no-relative-monorepo-imports
+
+Forbid relative imports that reach outside of the package in a monorepo.
+
+## Usage
+
+Add the rules as follows, it has no options:
+
+```js
+"@backstage/no-relative-monorepo-imports": ["error"]
+```
+
+The following patterns are considered files used during development, and only need dependencies to be declared in devDependencies:
+
+```python
+!src/**  # Any files outside of src are considered dev files
+src/**/*.test.*
+src/**/*.stories.*
+src/**/__testUtils__/**
+src/**/__mocks__/**
+src/setupTests.*
+```
+
+## Rule Details
+
+Assuming an import from for example `plugins/bar/src/index.ts`:
+
+### Fail
+
+```ts
+import { FooCard } from '../../foo';
+
+import { FooCard } from '../../foo/src/components/FooCard';
+```
+
+### Pass
+
+```ts
+import { FooCard } from '@internal/plugin-foo';
+
+// This is allowed by this rule, but not by no-forbidden-package-imports
+import { FooCard } from '@internal/plugin-foo/src/components/FooCard';
+```