update PR template about compiler warnings

Author: pcarruscag

.github/pull_request_template.md

@@ -12,7 +12,7 @@
 *Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These are a guide for you to know what the reviewers will be looking for in your contribution.*
 
 - [ ] I am submitting my contribution to the develop branch.
-- [ ] My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags).
+- [ ] My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
 - [ ] My contribution is commented and consistent with SU2 style.
 - [ ] I have added a test case that demonstrates my contribution, if necessary.
 - [ ] I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.