Adding an ISSUE_TEMPLATE.md (#4906)

* Adding an ISSUE_TEMPLATE.md

* imply -> rely on

* Fix typo in ISSUE_TEMPLATE

ISSUE_TEMPLATE.md

@@ -0,0 +1,23 @@
+- [ ] I'm reporting a bug, not asking for help
+- [ ] I've looked at the [documentation](http://leafletjs.com/reference-1.0.0.html) to make sure the behaviour is documented and expected
+- [ ] I'm sure this is a Leaflet code issue, not an issue with my own code nor with the framework I'm using (Cordova, Ionic, Angular, React…)
+- [ ] I've searched through the issues to make sure it's not yet reported
+
+----
+
+## How to reproduce
+
+- Leaflet version I'm using:
+- Browser (with version) I'm using:
+- OS/Platform (with version) I'm using:
+- step 1
+- step 2
+
+## What behaviour I'm expecting and which behaviour I'm seeing
+
+## Minimal example reproducing the issue
+
+- [ ] this example is as simple as possible
+- [ ] this example does not rely on any third party code
+
+Using http://playground-leaflet.rhcloud.com/ or any other jsfiddle like site.