Add issue template for new membership nominations.

Signed-off-by: James Blair <mail@jamesblair.net>

.github/ISSUE_TEMPLATE/membership-request.yml

@@ -0,0 +1,30 @@
+name: Membership nomination
+description: Nominate new etcd members
+labels:
+- area/community
+body:
+- type: textarea
+  id: feature
+  attributes:
+    label: Who would you like to nominate?
+  validations:
+    required: true
+
+- id: requirements
+  type: checkboxes
+  attributes:
+    label: Requirements
+    options:
+    - label: I have reviewed the [community membership guidelines](https://github.com/etcd-io/etcd/blob/main/Documentation/contributor-guide/community-membership.md)
+      required: true
+    - label: The members are actively contributing to 1 or more etcd subprojects
+      required: true
+    - label: The members are being sponsored by two current reviewers or a current maintainer.
+      required: true
+
+- type: textarea
+  id: rationale
+  attributes:
+    label: How do the new members meet the regular active contribution requirements?
+  validations:
+    required: true