Add placeholders in the product submission form

Add placeholders for some of the required inputs in the form. This will
be most helpful for the product version, but for completeness they have
also been added for the product vendor and name.

Co-authored-by: dav3r <david.redmin@trio.dhs.gov>

.github/ISSUE_TEMPLATE/product-submission-form.yml

@@ -8,6 +8,7 @@ body:
     attributes:
       label: Product vendor
       description: Who is the vendor for the product?
+      placeholder: Cisco, Dell, IBM, etc.
     validations:
       required: true
   - type: input
@@ -15,6 +16,7 @@ body:
     attributes:
       label: Product name
       description: What is the name of the product?
+      placeholder: AppDynamics, BigFix Inventory, Centera, etc.
     validations:
       required: true
   - type: input
@@ -22,6 +24,7 @@ body:
     attributes:
       label: Product version(s)
       description: What version(s) of the product are affected?
+      placeholder: v2; 1.5; >3; >=4; >5, <6; etc.
     validations:
       required: true
   - type: dropdown