KNOWN_BUGS: OS400 port requires deprecated IBM library

Closes #5176

docs/KNOWN_BUGS

@@ -46,6 +46,7 @@ problems may have been fixed or changed somewhat since this was written!
  4.4 Improve --data-urlencode space encoding
 
  5. Build and portability issues
+ 5.1 OS400 port requires deprecated IBM library
  5.2 curl-config --libs contains private details
  5.3 curl compiled on OSX 10.13 failed to run on OSX 10.10
  5.4 Build with staticly built dependency
@@ -344,6 +345,14 @@ problems may have been fixed or changed somewhat since this was written!
 
 5. Build and portability issues
 
+5.1 OS400 port requires deprecated IBM library
+
+ curl for OS400 requires QADRT to build, which provides ASCII wrappers for
+ libc/POSIX functions in the ILE, but IBM no longer supports or even offers
+ this library to download.
+
+ See https://github.com/curl/curl/issues/5176
+
 5.2 curl-config --libs contains private details
 
  "curl-config --libs" will include details set in LDFLAGS when configure is