Bucket limit text for explainer (#113)

* Add text about bucket limits for explainer

* Address comment

* Address comment

explainer.md

@@ -44,6 +44,7 @@
   - [Bucket names](#bucket-names)
   - [Storage policy naming](#storage-policy-naming)
   - [Default bucket quota](#default-bucket-quota)
+  - [Limit on number of buckets](#limit-on-number-of-buckets) 
 - [Considered alternatives](#considered-alternatives)
   - [Expose the API off of navigator.storage.buckets](#expose-the-api-off-of-navigatorstoragebuckets)
   - [Opening and creating buckets](#opening-and-creating-buckets)
@@ -611,6 +612,15 @@ disk space. However, the Chrome team thinks that having anything under
 the use of buckets. The Chrome team thinks developers should be able to use
 all available quota for an origin in one storage bucket.
 
+
+### Limit on Number of Buckets
+
+Bucket limits will be dynamically calculated base on the amount of quota available for the origin.
+User agents are expected to support a minumum of 10 buckets for each origin, and allow at least 1
+additional bucket for every 10 MiB of quota provided with a hard limit of 10,000 buckets.
+Attempting to create buckets in excess of this limit will result in a `QuotaExceededError`. 
+
+
 ## Privacy and Security considerations
 
 The Storage Buckets API and its implementation do not introduce any new security concerns. This feature