templates: added issue template

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,20 @@
+#### System information
+
+Geth version: `geth version`
+OS & Version: Windows/Linux/OSX
+Commit hash : (if `develop`)
+
+#### Expected behaviour
+
+
+#### Actual behaviour
+
+
+#### Steps to reproduce the behaviour
+
+
+#### Backtrace
+
+````
+[backtrace]
+````