Revise changelog for v0.5.6 (#50)

Author: laurenzlong

changelog.txt

@@ -1,3 +1,4 @@
 important - Update firebase-admin peer dependency to v4.2.1. Developer needs to run “npm install --save firebase-admin”  inside the functions directory.
 fixed - event.data.val() no longer returns array of nulls when the database node has a “length” key.
 fixed - Fixed bug where event.data.forEach(snapshot => {}) caused snapshot.key to be a concatenation of the parent and child keys.
+fixed - Fixed bug where nested storage objects had literal slashes in mediaLink property, instead of URL-escaped slashes.