Update V3 to V4 docs

doc/api/README.md

@@ -49,6 +49,7 @@ following locations:
 - [Todos](todos.md)
 - [Users](users.md)
 - [Validate CI configuration](ci/lint.md)
+- [V3 to V4](v3_to_v4.md)
 - [Version](version.md)
 
 ### Internal CI API

doc/api/v3_to_v4.md

+# V3 to V4 version
+
+Our V4 API version is currently available as *Beta*! It means that V3
+will still be supported and remain unchanged for now, but be aware that the following
+changes are in V4:
+
+### Changes
+
+- Removed `/projects/:search` (use: `/projects?search=x`)
+