[web-share-target] Introduce Level 2 Spec (#71)

Level 2 will add support for file sharing.

Author: ewilligers

.pr-preview.json

@@ -1,4 +1,4 @@
 {
-    "src_file": "index.html",
+    "src_file": "level-2/index.html",
     "type": "respec"
 }

README.md

@@ -6,7 +6,8 @@ This site details a proposal to add a capability for websites to receive shared
 data from other sites and apps.
 
 * [Explainer document](docs/explainer.md), a high-level overview of the proposal.
-* [Specification](https://wicg.github.io/web-share-target/).
+* [Specification - Level 1](https://wicg.github.io/web-share-target/).
+* [Specification - Level 2](https://wicg.github.io/web-share-target/level-2/).
 * [Native integration survey](docs/native.md), for platform-specific matters.
 
 This is a product of the [Ballista