Add complex attribute support

This change moves our `quill-delta` dependency onto our own forked
version, which has support for [complex attributes][1], which we need
for track changes.

[1]: slab/delta#48

.travis.yml

@@ -1,4 +1,4 @@
 language: node_js
-node_js: '6'
+node_js: '10'
 script:
   - npm test

README.md

@@ -1,12 +1,19 @@
 # Rich Text OT Type [![Build Status](https://travis-ci.org/ottypes/rich-text.svg?branch=master)](https://travis-ci.org/ottypes/rich-text)
 
-## Why fork?
+## Reedsy: Why fork?
 
-We're running on a fork of the original [`rich-text` operational transform type](https://github.com/ottypes/rich-text). This is because we have a need to store metadata on our rich text documents so that we can determine which book a given piece of content belongs to. We need to know this so that we can validate a user's access to the content in middleware.
+We need the following features on top of the original `rich-text` type:
 
-The alternative to this solution would have been to store a separate Mongo document that maps a book to all the chapters it owns, requiring a database lookup every time we submit an op, which would be quite heavy. This could naturally be eased a bit with caching, but cache invalidation is generally non-trivial, and storing metadata on the chapter itself feels more "natural".
+  - additional metadata fields
+  - support for complex attributes
 
-The result is an extremely small wrapper around the original type, which stores a metadata object on the document. Note that intermittent deltas should remain entirely unaffected, and it will only be stored on creation, and restored on fetch.
+### Metadata fields
+
+In order to perform authentication on our documents, the documents must have a concept of which book they belong to. We add this into an additional metadata field, which the original `rich-text` type does not support.
+
+### Complex attributes
+
+Attributes are typically only "1 layer" deep. We need arbitrarily-nested attributes, which can be partially updated, in order to support track changes. For more details, see [this pull request](https://github.com/quilljs/delta/pull/48).
 
 ## Introduction