Update contributing guidelines to highlight maintenance cost

.github/PULL_REQUEST_TEMPLATE/new-feature.md

@@ -0,0 +1,23 @@
+### Description
+
+### Maintenance cost vs benefit
+
+<!--
+As the project maintainer, I have to carefully balance new features with the limited time available for maintenance.
+While we highly value contributions, new features grow the codebase and increase the ongoing maintenance burden - so
+consider benefit vs maintenance cost - see https://github.com/scanamo/scanamo/blob/main/CONTRIBUTING.md#new-features
+for more details.
+-->
+
+* **Benefit**: <!-- eg, adds the capability to do X, better (in what ways?) than directly using the AWS SDK -->
+* **Cost**: <!-- eg, adds this much extra implementation code, modifies this many files, breaks compatibility, etc -->
+
+### Tests
+
+<!--
+Any new functionality should have at least one test, both to check correctness, and to
+demonstrate API use.
+-->
+
+### See also
+<!-- GitHub Issues, PRs (including Scanamo PRs that have previously touched this area), AWS documentation, etc -->

CONTRIBUTING.md

@@ -1,17 +1,44 @@
-If you have a question about Scanamo there is a 
-[Gitter channel](https://gitter.im/guardian/scanamo) to try and 
-answer it. Suggestions on how to improve the documentation are very
-welcome.
+## Issues & Questions
 
-Feel free to open an issue if you notice a bug or have an idea for a
-feature. 
+If you've found what looks like a bug, please check [issues on GitHub](https://github.com/scanamo/scanamo/issues),
+and create a new issue if necessary.
 
-Pull requests are gladly accepted. Scanamo follows a standard
-[fork and pull](https://help.github.com/articles/using-pull-requests/)
-model for contributions via GitHub pull requests.
+If you just have a general question, or there's something you don't understand, ask on
+[stackoverflow.com](http://stackoverflow.com/questions/ask) (tag it with [`scanamo`](http://stackoverflow.com/questions/tagged/scanamo)
+so we can see it) - there are many more people who can answer that sort of question on Stackoverflow,
+you stand a good chance of getting your question answered quicker!
 
-Building and testing Scanamo
-----------------------------
+There _was_ a [Gitter channel](https://gitter.im/guardian/scanamo) for discussion/questions around Scanamo, but
+it's not a preferred channel for the current maintenance team, and isn't checked very often.
+
+## Pull requests
+
+### Documentation
+
+Suggestions on how to improve the documentation are very welcome!
+
+### New features
+
+New features inevitably grow the Scanamo codebase, unfortunately increasing its ongoing
+maintenance burden. As maintenance time is precious and in short supply, new features need
+to justify themselves in terms of benefit-to-users versus cost-in-maintenance - maintenance
+cost can be roughly correlated to how much implementation code is added, as an example, see
+[PR #1801](https://github.com/scanamo/scanamo/pull/1801#pullrequestreview-2258441456).
+
+New features that don't significantly improve on just-directly-using-the-AWS-SDK stand an increased
+risk of being rejected. Of the ~60 operations supported by the [DynamoDB API](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Operations_Amazon_DynamoDB.html),
+only 9 are directly supported by Scanamo - for each supported operation, the benefits are
+usually _more_ than one from this list:
+
+* **conciseness** - accomplishing tasks with less code than directly using the AWS SDK
+* **preventing the construction of invalid requests** (eg, `expressionAttributeNames` & `expressionAttributeValues` correctly set, avoiding reserved words, etc)
+* **facilitating mapping** - between Scala/Java data types/classes and the DynamoDB item attributes model
+
+Scanamo's maintainers have little opportunity to see how others use Scanamo - it may
+help maintainers understand benefit if you can describe a specific example of how you intend to use
+the feature (the best example would be linking to a draft public PR that uses the feature!).
+
+## Building and testing Scanamo
 
 Scanamo uses a standard [SBT](https://www.scala-sbt.org/) build. If you
 have SBT installed, you should first run `startDynamodbLocal` task from the SBT prompt to start a local dynamodb instance and afterwards run the `test` command to compile Scanamo and run its tests.
@@ -33,7 +60,7 @@ scalafmtCheck
 scalafmtSbtCheck
 ```
 
-# Publishing a new release
+## Publishing a new release
 
 This repo uses [`gha-scala-library-release-workflow`](https://github.com/guardian/gha-scala-library-release-workflow)
 to automate publishing releases (both full & preview releases) - see