Check that develop is target in PR

This is an update to the PR template.
It mostly adds text and some checks related to how good the template
is filled in and whether the `develop` branch was selected as base.

Author: cimnine

.github/pull_request_template.md

@@ -1,21 +1,40 @@
 <!--
-Please don't open an issue when submit PR.
+###############################################################################
+
+Thank you for sharing your work and for opening a PR.
+
+(!) IMPORTANT (!):
+First make sure that you point your PR to the `develop` branch!
+
+Now please read the comments carefully and try to provide information
+on all relevant titles.
+
+###############################################################################
+-->
+
+<!--
+Please don't open an extra issue when submiting a PR.
+
+But if there is already a related issue, please put it's number here.
+
+E.g. #123 or N/A
+-->
 
-But if there is already a related issue, please put it's number here. E.g. #123 or N/A -->
 Related Issue:
 
 ## New Behavior
 
 <!--
-please describe in a few words the behavior your PR implements
+Please describe in a few words the intentions of your PR.
 -->
 
 ...
 
 ## Contrast to Current Behavior
 
 <!--
-please describe in a few words how the new behavior is different from the current behavior
+Please describe in a few words how the new behavior is different
+from the current behavior.
 -->
 
 ...
@@ -24,12 +43,15 @@ please describe in a few words how the new behavior is different from the curren
 
 <!--
 Please make your case here:
-- Why do you think this project and the community will benefit from your proposed change?
+
+- Why do you think this project and the community will benefit from your
+  proposed change?
 - What are the drawbacks of this change?
 - Is it backwards-compatible?
 - Anything else that you think is relevant to the discussion of this PR.
 
-(No need to write a huge article here. Just a few sentences that give some additional context about the motivations for the change.)
+(No need to write a huge article here. Just a few sentences that give some
+additional context about the motivations for the change.)
 -->
 
 ...
@@ -38,7 +60,17 @@ Please make your case here:
 
 <!--
 If the README.md must be updated, please include the changes in the PR.
-If the Wiki must be updated, please make a suggestions below.
+If the Wiki must be updated, please make a suggestion below.
 -->
 
 ...
+
+## Double Check
+
+<!--
+Please put an x into the brackets (like `[x]`) if you've completed that task.
+-->
+
+* [ ] I have read the comments and followed the PR template.
+* [ ] I have provided a explained my PR according to the information in the comments.
+* [ ] My PR targets the `develop` branch.