document use of COMPOSE_PROJECT_NAME as env variable

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

spec.md

@@ -231,6 +231,18 @@ Top-level `name` property is defined by the specification as project name to be
 Compose implementations MUST offer a way for user to override this name, and SHOULD define a mechanism to compute a
 default project name, to be used only if the top-level `name` element is not set.
 
+Whenever project name is defined by top-level `name` or by some custom mechanism, it MUST be exposed for 
+[interpolation](#Interpolation) and environment variable resolution as `COMPOSE_PROJECT_NAME`
+
+```yml
+services:
+  foo:
+    image: busybox
+    environment:
+      - COMPOSE_PROJECT_NAME
+    command: echo "I'm running ${COMPOSE_PROJECT_NAME}"
+```
+
 ## Services top-level element
 
 A Service is an abstract definition of a computing resource within an application which can be scaled/replaced