Replace template README, add sample welcome campaign

Matches the pattern already in place for the other domain repos: a
short domain-specific README instead of the generic campaigns-template
one (also fixing a stray "from" -> "from_email" field-name error), and
a sample welcome campaign using the correct "cateconners-list" name
from the start.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

campaigns/welcome/campaign.md

@@ -0,0 +1,16 @@
+---
+subject: "Welcome!"
+lists: ["cateconners-list"]
+from_email: "Cate Conners <hello@cateconners.com>"
+tags: ["welcome"]
+---
+
+{{ if .Subscriber.Attribs.first_name }}Hi {{ .Subscriber.Attribs.first_name }},{{ else }}Hi there,{{ end }}
+
+Welcome, and thanks for subscribing!
+
+From now on, this is where you'll hear from me directly — new work, updates,
+and anything else worth sharing. No spam, no noise, just the good stuff.
+
+Talk soon,
+Cate