Add membership issue template

.github/ISSUE_TEMPLATE/membership.md

@@ -0,0 +1,29 @@
+---
+name: Organization Membership Request
+about: Request membership in Carvel Org
+title: 'REQUEST: New membership for <your-GH-handle>'
+labels: area/github-membership
+assignees: ''
+
+---
+
+### GitHub Username
+
+@<your-GH-handle>
+
+### Requirements
+- [ ] I have reviewed [the community membership guidelines](https://github.com/carvel-dev/carvel/blob/develop/processes/community-membership.md)
+- [ ] I have [enabled 2FA on my GitHub account](https://github.com/settings/security)
+- [ ] I am actively contributing to 1 or more Carvel subprojects
+- [ ] I have two sponsors that meet the sponsor requirements listed in the community membership guidelines
+- [ ] I have spoken to my sponsors ahead of this application, and they have agreed to sponsor my application
+- [ ] I understand that I can [make my membership public](https://docs.github.com/en/account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-your-membership-in-organizations/publicizing-or-hiding-organization-membership) if I'd like to once I am invited to the organization
+
+### Sponsors
+- @<sponsor-1>
+- @<sponsor-2>
+
+### List of contributions to the Carvel project
+- PRs reviewed / authored
+- Issues responded to
+- Projects I am involved with