Added GitHub issue and PR templates

GitHub will use these templates to pre-fill created issues and
pull requests. This should help to get people to provide better
information on issues or direct them to the forum when needed and
hint people to provide example/test code for pull requests.

.github/ISSUE_TEMPLATE.md

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+Thanks for raising your issue here! 🙂
+
+Before you submit the issue however, we'd like you to consider the follow points.
+
+* We like to use the issue tracker for confirmed issues.
+* If you're stuck with SFML, please use [the forum](https://en.sfml-dev.org/forums/index.php#c3) to get help.
+
+----
+
+## Subject of the issue
+
+Describe your issue here.
+
+## Your environment
+
+* Your OS / distro / window manager used
+* Your version of SFML (2.5.0, git master, etc)
+* Your compiler and compiler version used
+* Special compiler flags used
+
+## Steps to reproduce
+
+Tell us how to reproduce this issue. Please provide a [minimal, complete and verifiable example](https://stackoverflow.com/help/mcve), you can use the follow template as a start:
+
+```cpp
+#include <SFML/Graphics.hpp>
+
+int main()
+{
+    sf::RenderWindow window(sf::VideoMode(1280, 720), "Minimal, complete and verifiable example");
+    window.setFramerateLimit(60);
+
+    while (window.isOpen())
+    {
+        sf::Event event;
+        while (window.pollEvent(event))
+        {
+            if (event.type == sf::Event::Closed)
+                window.close();
+        }
+
+        window.clear();
+        window.display();
+    }
+}
+```
+
+## Expected behavior
+
+Tell us what should happen
+
+## Actual behavior
+
+Tell us what happens instead

.github/PULL_REQUEST_TEMPLATE.md

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+Thanks a lot for making a contribution to SFML! 🙂
+
+Before you create the pull request, we ask you to check the follow boxes. (For small changes not everything needs to ticked, but the more the better!)
+
+* [ ] Has this change been discussed on [the forum](https://en.sfml-dev.org/forums/index.php#c3) or in an issue before?
+* [ ] Does the code follow the SFML [Code Style Guide](https://www.sfml-dev.org/style.php)?
+* [ ] Have you provided some example/test code for your changes?
+* [ ] If you have additional steps which need to be performed list them as tasks!
+
+----
+
+## Description
+
+Please describe your pull request.
+
+This PR is related to the issue #
+
+## Tasks
+
+* [ ] Tested on Linux
+* [ ] Tested on Windows
+* [ ] Tested on macOS
+* [ ] Tested on iOS
+* [ ] Tested on Android
+
+## How to test this PR?
+
+Describe how to best test these changes. Please provide a [minimal, complete and verifiable example](https://stackoverflow.com/help/mcve) if possible, you can use the follow template as a start:
+
+```cpp
+#include <SFML/Graphics.hpp>
+
+int main()
+{
+    sf::RenderWindow window(sf::VideoMode(1280, 720), "Minimal, complete and verifiable example");
+    window.setFramerateLimit(60);
+
+    while (window.isOpen())
+    {
+        sf::Event event;
+        while (window.pollEvent(event))
+        {
+            if (event.type == sf::Event::Closed)
+                window.close();
+        }
+
+        window.clear();
+        window.display();
+    }
+}
+```