docs: Add pull request template

Author: deandreamatias

.github/ISSUE_TEMPLATE/custom.md

@@ -0,0 +1,21 @@
+## Connection with issue(s)
+
+<!-- If this pull request close some issue, use this reference to close it automatically -->
+Close #???
+
+<!-- Optional: other issues or pull requests related to this, but merging should not close it -->
+Connected to #???
+
+## Solution description
+
+## Screenshots or Videos
+
+<!-- Optional: to clearly demonstrate the feature or fix to help with testing and reviews -->
+
+## To Do
+
+- [ ] Read [contributing guide](https://github.com/flutter-form-builder-ecosystem/form_builder_file_picker/blob/master/CONTRIBUTING.md)
+- [ ] Check the original issue to confirm it is fully satisfied
+- [ ] Add solution description to help guide reviewers
+- [ ] Add unit test to verify new or fixed behaviour
+- [ ] If apply, add documentation to code properties and package readme