MyFirstContribution: add walken.c to meson.build

Instruct in the documentation to also add an entry in meson.build for
builtin/walken.c, as currently both Meson and Make are supported.

Helped-by: Karthik Nayak <[email protected]>
Helped-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Lucas Seiki Oshiro <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: lucasoshiro

Documentation/MyFirstObjectWalk.adoc

@@ -96,10 +96,23 @@ maintaining alphabetical ordering:
 { "walken", cmd_walken, RUN_SETUP },
 ----
 
-Add it to the `Makefile` near the line for `builtin/worktree.o`:
+Add an entry for the new command in the both the Make and Meson build system,
+before the entry for `worktree`:
 
+- In the `Makefile`:
 ----
+...
 BUILTIN_OBJS += builtin/walken.o
+...
+----
+
+- In the `meson.build` file:
+----
+builtin_sources = [
+   ...
+  'builtin/walken.c',
+   ...
+]
 ----
 
 Build and test out your command, without forgetting to ensure the `DEVELOPER`