feat: Add 'Alternatives Considered' section to RFDs

Per issue #431, this adds a new section to the RFD template for
RFD authors to describe any alternative paths considered and
provide reasons why those paths are not being pursued. This is
intended to help reviewers for RFDs, and to provide useful context
for anyone looking back at RFDs in the future.

Signed-off-by: Andrew Lilley Brinker

Author: alilleybrinker

rfds/_TEMPLATE.md

@@ -141,6 +141,16 @@ in the prescribed time.
 Describe any evidence for the need to adopt the RFD proposal based on
 community demand for specific new data or demand for better data quality.
 
+## Alternatives Considered
+[alternatives-considered]: #alternatives-considered
+
+Describe any alternative options considered during the formulation of this
+proposal, including any major reasons why those options are not being pursued.
+This does not need to be a complete breakdown of the details of each alternate
+option, but should provide enough detail that reviewers and future contributors
+can understand the major motivations behind pursuing the recommended path
+versus the alternatives.
+
 ## Related Issues or Proposals
 [related-issues-or-proposals]: #related-issues-or-proposals