FINERACT-2416: improve PR template

I started editing this just to fix the link to the "Pull Requests" doc (it moved from README.md to CONTRIBUTING.md), but then I saw a few other places in need of service.

* use Markdown links for readability
* add link to pull request size limit wiki page re: code dumps
* remove link to PR 1284, it doesn't add anything here and takes the reader out of context
* add link to mailing list
* add link to testing docs

Author: meonkeys

.github/pull_request_template.md

@@ -1,18 +1,16 @@
 ## Description
 
-Describe the changes made and why they were made.
-
-Ignore if these details are present on the associated [Apache Fineract JIRA ticket](https://github.com/apache/fineract/pull/1284).
+Describe the changes made and why they were made. (Ignore if these details are present on the associated Apache Fineract JIRA ticket.)
 
 ## Checklist
 
 Please make sure these boxes are checked before submitting your pull request - thanks!
 
-- [ ] Write the commit message as per https://github.com/apache/fineract/#pull-requests
+- [ ] Write the commit message as per [our guidelines](https://github.com/apache/fineract/blob/develop/CONTRIBUTING.md#pull-requests)
 - [ ] Acknowledge that we will not review PRs that are not passing the build _("green")_ - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
-- [ ] Create/update unit or integration tests for verifying the changes made.
-- [ ] Follow coding conventions at https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions.
+- [ ] Create/update [unit or integration tests](https://fineract.apache.org/docs/current/#_testing) for verifying the changes made.
+- [ ] Follow our [coding conventions](https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions).
 - [ ] Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
-- [ ] Submission is not a "code dump".  (Large changes can be made "in repository" via a branch.  Ask on the developer mailing list for guidance, if required.)
+- [ ] [This PR must not be a "code dump"](https://cwiki.apache.org/confluence/display/FINERACT/Pull+Request+Size+Limit). Large changes can be made in a branch, with assistance. Ask for help on the [developer mailing list](https://fineract.apache.org/#contribute).
 
-FYI our guidelines for code reviews are at https://cwiki.apache.org/confluence/display/FINERACT/Code+Review+Guide.
+Your assigned reviewer(s) will follow our [guidelines for code reviews](https://cwiki.apache.org/confluence/display/FINERACT/Code+Review+Guide).