Update transform-es2015 READMEs from babel.github.io [skip ci] (#4926)

Signed-off-by: Brian Ng <bng412@gmail.com>

packages/babel-plugin-transform-es2015-computed-properties/README.md

@@ -1,6 +1,6 @@
 # babel-plugin-transform-es2015-computed-properties
 
-Compile ES2015 computed properties to ES5
+> Compile ES2015 computed properties to ES5
 
 ## Installation
 
@@ -43,3 +43,9 @@ require("babel-core").transform("code", {
   plugins: ["transform-es2015-computed-properties"]
 });
 ```
+
+## Options
+
+* `loose` - Just like method assignment in classes, in loose mode, computed property names
+use simple assignments instead of being defined. This is unlikely to be an issue
+in production code.