doc: update COMPILE (clang-6 works)
@@ -122,6 +122,7 @@ https://build.libelektra.org/
| mingw | 4.6 | i386 |
| clang | 3.8 |x86_64-pc-linux-gnu|
| clang | 5.0 |x86_64-pc-linux-gnu|
+| clang | 6.0 |x86_64-pc-linux-gnu|
| clang | 8.1.0 | macOS |
| icc | 14.0.2 20140120 |x86_64-pc-linux-gnu|
| gcc/g++ | | openbsd 4.9.3 (*) |
