Add SUPPORT.md

Just replicating the "Maintenance and support guidelines" text from
<https://unit.nginx.org/community/>. With a link to it from the README.

Cc: Maryna Herasimovich <[email protected]
Signed-off-by: Andrew Clayton <[email protected]>

Author: ac000

README.md

@@ -8,6 +8,8 @@
 
 ![NGINX Unit Logo](docs/unitlogo.svg)
 
+Please see: [SUPPORT.md](/SUPPORT.md)
+
 NGINX Unit is a lightweight and versatile open-source server that has
 two primary capabilities:
 

SUPPORT.md

@@ -0,0 +1,10 @@
+# Maintenance and support guidelines
+
+Going forward, NGINX Unit will no longer receive new features or
+enhancements from the project maintainers. NGINX Unit is still open to
+community contributions.
+
+The team is focusing its limited capacity to keep NGINX Unit stable,
+secure and high-quality. Its focus will be on critical maintenance, such
+as for security issues, dependency upgrades, and resolving crashes or
+undefined behavior.