doc CHANGE add infor about missing extensions plugins into FAQ

Author: rkrejci

FAQ.md

@@ -21,4 +21,23 @@ $ make
    or add the libyang's location to the linker paths in `/etc/ld.so.conf.d` and
    then run `ldconfig` to rebuild the linker cache.
 
+__Q: yanglint(1) does not start and, but prints the following error messages:__
+```
+./yanglint
+libyang[0]: Invalid keyword "type" as a child to "annotation". (path: /)
+libyang[0]: Module "yang" parsing failed.
+Failed to create context.
+```
+
+__A:__ To handle complex YANG extensions, libyang (and therefore yanglint(1))
+   needs plugins. By default, the plugins are installed into the system path
+   (next to the libyang library into the separate `libyang` subdirectory). If
+   libyang was not installed, yanglint cannot find these plugins and it fails.
+   If you do not want to install libyang, it is possible to specify path to the
+   plugins via environment variable. The plugins can be found in the libyang
+   build directory in `src/extensions/` subdirectory. So running yanglint(1)
+   then can be made this way:
+```
+$ LIBYANG_EXTENSIONS_PLUGINS_DIR=`pwd`/src/extensions ./yanglint
+```