contributing: clarify scope (#993)

docs/contributing.rst

@@ -61,6 +61,35 @@ Outside of data compatibility, follow the usual semver rules for API changes and
 to a major release.
 
 
+The Scope of the Library
+------------------------
+
+The purpose of this library is to provide a Pythonic ODM layer on top of DynamoDB to be used
+in server applications' runtime, i.e. to enable their various application logic and features.
+While striving for the library to be useful, we're also trying to "do one thing well". For this reason:
+
+- Database administration tasks are out of scope, and while PynamoDB has functions for
+  operations like CreateTable, CreateIndex and DeleteTable, it's because they are useful
+  for interacting with dynamodb-local and moto's DynamoDB backend from within test code.
+
+  For this reason, features such as enabling PITR backups, restoring from such backups,
+  updating indices, etc. are intentionally absent. For getting started and operating
+  on a small scale, AWS Console and the AWS Command Line Interface (awscli) can be used.
+  For larger scale, infrastructure provisioning by dedicated tools (such as CloudFormation
+  or Terraform) would be vastly preferable over anything PynamoDB could offer.
+
+  Per security best practices, we recommend running your application's runtime with an IAM role
+  having the least privileges necessary for it to function (which likely excludes any database
+  administration operations).
+
+- While the library aims to empower application developers, it steers away from high-level features
+  which are not specific to DynamoDB. For example, a custom attribute which serializes UUIDs
+  as strings is doubtlessly something many applications have had a need for, but as long as it doesn't
+  exercise any core DynamoDB functionality (e.g. in the case of a UUID attribute, there isn't
+  a dedicated DynamoDB data type or API feature for storing UUIDs), we would recommend relegating
+  such functionality to auxillary libraries. One such library is `pynamodb-attributes <https://github.com/lyft/pynamodb-attributes>`_.
+
+
 Pull Requests
 -------------