Update HACKING regarding adding a lexer and using meson

HACKING

@@ -525,7 +525,7 @@ printf("Prefix: %s", GEANY_PREFIX);
 
 Adding a source file foo.[hc] in src/ or plugins/
 -------------------------------------------------
-* Add foo.c, foo.h to SRCS in path/Makefile.am.
+* Add foo.c, foo.h to SRCS in path/Makefile.am and meson.build.
 * Add path/foo.c to po/POTFILES.in (for string translation).
 
 Adding a filetype
@@ -585,10 +585,12 @@ LexFoo.cxx. Try the official Scintilla project first.
     Scintilla. All new lexers should be submitted back to the Scintilla
     project to save duplication of work.
 
-When adding a lexer, update:
+When adding a lexer:
 
-* scintilla/Makefile.am
-* scintilla/src/Catalogue.cxx - add a LINK_LEXER command *manually*
+* add LexFoo.cxx to scintilla/lexilla/lexers
+* add `&lmFoo` to scintilla/lexilla/src/Lexilla.cxx and
+  scintilla/scintilla_changes.patch
+* update scintilla/Makefile.am and meson.build
 
 For syntax highlighting, you will need to edit highlighting.c and
 highlightingmappings.h and add the following things:
@@ -659,7 +661,7 @@ https://github.com/universal-ctags/ctags
 
 Method
 ``````
-* Add foo.c to SRCS in Makefile.am.
+* Add foo.c to SRCS in Makefile.am and meson.build.
 * Add Foo to src/tagmanager/tm_parsers.h
 * Add TM_PARSER_FOO to src/tagmanager/tm_parser.h.  The list here must follow
   exactly the order in src/tagmanager/tm_parsers.h.
@@ -704,8 +706,8 @@ the tag information expected to be generated from the corresponding
 source file.
 
 When you have these two files, you have to list your new test along the
-other ones in the ``test_source`` variable in ``tests/ctags/Makefile.am``.
-Please keep this list sorted alphabetically.
+other ones in the ``test_source`` variable in ``tests/ctags/Makefile.am``
+and ``tests/meson.build``. Please keep this list sorted alphabetically.
 
 Upgrading Scintilla
 -------------------