[Docs] no-unused-modules: Indicates usage, plugin defaults to no-op, and add description to main README.md

Fixes #1351

Author: johndevedu

README.md

@@ -53,13 +53,15 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 * Report imported names marked with `@deprecated` documentation tag ([`no-deprecated`])
 * Forbid the use of extraneous packages ([`no-extraneous-dependencies`])
 * Forbid the use of mutable exports with `var` or `let`. ([`no-mutable-exports`])
+* Report modules without exports, or exports without matching import in another module ([`no-unused-modules`])
 
 [`export`]: ./docs/rules/export.md
 [`no-named-as-default`]: ./docs/rules/no-named-as-default.md
 [`no-named-as-default-member`]: ./docs/rules/no-named-as-default-member.md
 [`no-deprecated`]: ./docs/rules/no-deprecated.md
 [`no-extraneous-dependencies`]: ./docs/rules/no-extraneous-dependencies.md
 [`no-mutable-exports`]: ./docs/rules/no-mutable-exports.md
+[`no-unused-modules`]: ./docs/rules/no-unused-modules.md
 
 ### Module systems
 

docs/rules/no-unused-modules.md

@@ -8,15 +8,26 @@ Note: dynamic imports are currently not supported.
 
 ## Rule Details
 
+### Usage
+
+In order for this plugin to work, one of the options `missingExports` or `unusedExports` must be enabled (see "Options" section below). In the future, these options will be enabled by default (see https://github.com/benmosher/eslint-plugin-import/issues/1324)
+
+Example: 
+```
+"rules: {
+  ...otherRules,
+  "import/no-unused-modules": [1, {"unusedExports": true}]
+}
+```
 
 ### Options
 
 This rule takes the following option:
 
+- **`missingExports`**: if `true`, files without any exports are reported (defaults to `false`)
+- **`unusedExports`**: if `true`, exports without any static usage within other modules are reported (defaults to `false`)
 - `src`: an array with files/paths to be analyzed. It only applies to unused exports. Defaults to `process.cwd()`, if not provided
 - `ignoreExports`: an array with files/paths for which unused exports will not be reported (e.g module entry points in a published package) 
-- `missingExports`: if `true`, files without any exports are reported  
-- `unusedExports`: if `true`, exports without any static usage within other modules are reported.
 
 
 ### Example for missing exports