doc: Check standalone compilation before submitting new components.

doc/developer.texi

@@ -299,8 +299,13 @@ send a reminder by email. Your patch should eventually be dealt with.
     configure?
 @item
     Did you @code{git add} the appropriate files before committing?
+@item
+    Did you make sure it compiles standalone, i.e. with
+    @code{configure --disable-everything --enable-decoder=foo}
+    (or @code{--enable-demuxer} or whatever your component is)?
 @end enumerate
 
+
 @section patch submission checklist
 
 @enumerate