Including link to Discourse

Adding link to Stan's GSoC discourse where interested students can ask questions/communicate with each other and project mentors.

Author: stablemarkets

gsoc_proposals/2021/proposal_main.md

@@ -4,3 +4,6 @@ Stan is excited to offer the following projects for GSoC 2021:
   - [Lambert W Transforms](lambertw.md)
   - [Benchmarking Bayesian Models in Stan](bayesian_benchmarking.md)
   - [Garch Models in BRMS](garch.md)
+
+If you have questions or would like to reach out to mentors, please post in this thread on Stan's Discourse: 
+https://discourse.mc-stan.org/t/gsoc-2021-q-a-thread/21261