Add first draft of GSOC issue template (#6865)

.github/ISSUE_TEMPLATE/GSOC.md

@@ -0,0 +1,28 @@
+---
+name: "Google Summer of Code"
+about: See https://github.com/wagtail/wagtail/wiki/Google-Summer-of-Code-2021
+title: ''
+labels: 'GSOC'
+assignees: thibaudcolas, dawnwages
+
+---
+
+## Summary 
+
+
+
+## Implementation
+
+
+
+## Skills
+
+
+
+## Mentors
+
+
+
+## Aims
+
+