Add explanation to README

Author: kemitchell

README.md

@@ -1,8 +1,26 @@
+This package parses SPDX license expression strings describing license terms, like [package.json license strings](https://docs.npmjs.com/files/package.json#license), into consistently structured ECMAScript objects.  The npm command-line interface depends on this package, as do many automatic license-audit tools.
+
+In a nutshell:
+
 ```javascript
 var parse = require('spdx-expression-parse')
 var assert = require('assert')
 
 assert.deepEqual(
+  // Licensed under the terms of the Two-Clause BSD License.
+  parse('BSD-2-Clause'),
+  {license: 'BSD-2-Clause'}
+)
+
+assert.throws(function () {
+  // An invalid SPDX license expression.
+  // Should be `Apache-2.0`.
+  parse('Apache 2')
+})
+
+assert.deepEqual(
+  // Dual licensed under LGPL 2.1 or a combination of the Three-Clause
+  // BSD License and the MIT License.
   parse('(LGPL-2.1 OR BSD-3-Clause AND MIT)'),
   {
     left: {license: 'LGPL-2.1'},
@@ -14,32 +32,52 @@ assert.deepEqual(
     }
   }
 )
+```
 
-assert.deepEqual(
-  parse('(MIT AND (LGPL-2.1+ AND BSD-3-Clause))'),
-  {
-    left: {license: 'MIT'},
-    conjunction: 'and',
-    right: {
-      left: {license: 'LGPL-2.1', plus: true},
-      conjunction: 'and',
-      right: {license: 'BSD-3-Clause'}
-    }
-  }
-)
+The syntax comes from the [Software Package Data eXchange (SPDX)](https://spdx.org/), a standard from the [Linux Foundation](https://www.linuxfoundation.org) for shareable data about software package license terms.  SPDX aims to make sharing and auditing license data easy, especially for users of open-source software.
+
+The bulk of the SPDX standard describes syntax and semantics of XML metadata files.  This package implements two lightweight, plain-text components of that larger standard:
+
+1.  The [license list](https://spdx.org/licenses), a mapping from specific string identifiers, like `Apache-2.0`, to standard form license texts and bolt-on license exceptions.  The [spdx-license-ids](https://www.npmjs.com/package/spdx-exceptions) and [spdx-exceptions](https://www.npmjs.com/package/spdx-license-ids) packages implement the license list.  They are development dependencies of this package.
+
+    Any license identifier from the license list is a valid license expression:
+
+    ```javascript
+    require('spdx-license-ids').forEach(function (id) {
+      assert.deepEqual(parse(id), {license: id})
+    })
+    ```
 
-// We handle all the bare SPDX license and exception ids as well.
-require('spdx-license-ids').forEach(function (id) {
-  assert.deepEqual(parse(id), {license: id})
-  require('spdx-exceptions').forEach(function (e) {
+    So is any license identifier `WITH` a standardized license exception:
+
+    ```javascript
+    require('spdx-license-ids').forEach(function (id) {
+      require('spdx-exceptions').forEach(function (e) {
+        assert.deepEqual(
+          parse(id + ' WITH ' + e),
+          {license: id, exception: e}
+        )
+      })
+    })
+    ```
+
+2.  The license expression language, for describing simple and complex license terms, like `MIT` for MIT-licensed and `(GPL-2.0 OR Apache-2.0)` for dual-licensing under GPL 2.0 and Apache 2.0.  This package implements the license expression language.
+
+    ```javascript
     assert.deepEqual(
-      parse(id + ' WITH ' + e),
-      {license: id, exception: e}
+      // Licensed under a combination of the MIT License and a combination
+      // of LGPL 2.1 (or a later version) and the Three-Clause BSD License.
+      parse('(MIT AND (LGPL-2.1+ AND BSD-3-Clause))'),
+      {
+        left: {license: 'MIT'},
+        conjunction: 'and',
+        right: {
+          left: {license: 'LGPL-2.1', plus: true},
+          conjunction: 'and',
+          right: {license: 'BSD-3-Clause'}
+        }
+      }
     )
-  })
-})
-```
-
----
+    ```
 
-[The Software Package Data Exchange (SPDX) specification](http://spdx.org) is the work of the [Linux Foundation](http://www.linuxfoundation.org) and its contributors, and is licensed under the terms of [the Creative Commons Attribution License 3.0 Unported (SPDX: "CC-BY-3.0")](http://spdx.org/licenses/CC-BY-3.0). "SPDX" is a United States federally registered trademark of the Linux Foundation.
+The Linux Foundation and its contributors license the SPDX standard under the terms of [the Creative Commons Attribution License 3.0 Unported (SPDX: "CC-BY-3.0")](http://spdx.org/licenses/CC-BY-3.0).  "SPDX" is a United States federally registered trademark of the Linux Foundation.  The authors of this package license their work under the terms of the MIT License.