doc: added instructions for package override using local.mk

doc/developers-guide.md

@@ -75,7 +75,7 @@ To see available defconfigs for supported targets, use:
 Development
 -----------
 
-When changing a package, locally kept sources, or when using `local.mk`,
+When changing a package, locally kept sources, or when using [`local.mk`](override-package.md),
 you only want to rebuild the parts you have modified:
 
     make foo-rebuild

doc/override-package.md

@@ -0,0 +1,50 @@
+Package override
+================
+
+This guide demonstrates how the `local.mk` file is utilized to override 
+a Linux Buildroot package. The example of `tcpdump` serves to illustrate 
+this process.
+
+In an instance such as `Infix` using tcpdump, the `local.mk` file is modified 
+as shown below: 
+
+```
+TCPDUMP_OVERRIDE_SRCDIR = /path/to/tcpdump/repo
+```
+
+and stored in the `output/` folder, alongside the `.config` file:
+```
+user@PC:~/infix$ll output/ | grep -e 'local.mk' -e '.config'
+-rw-r--r--   1 group user 119936 Nov 10 18:04 .config
+-rw-r--r--   1 group user     43 Nov 10 18:25 local.mk
+```
+
+The execution of `make tcpdump-rebuild all` triggers a process where 
+Buildroot synchronizes the tcpdump source code from the specified override directory 
+to `output/build/tcpdump-custom`, followed by the rebuilding of the entire project. 
+
+```
+user@PC:~/infix$ make tcpdump-rebuild all
+```
+
+
+```
+user@PC:~/infix$ ll /output/build/ | grep tcpdump
+drwxr-xr-x   7 group user 20480 Nov 10 18:26 tcpdump-4.99.4/
+drwxr-xr-x   7 group user 12288 Nov 10 18:28 tcpdump-custom/
+```
+
+Buildroot follows a process of downloading and processing tarballs 
+(extraction, configuration, compilation, and installation). 
+The source code is stored  in a temporary directory:
+`output/build/<package>-<version>` (i.e. `tcpdump-4.99.4/`), 
+which is removed and recreated with each `make` command. That is why 
+the direct modifications in the `output/build` directory are generally 
+**not recommended**. 
+
+To manage the development changes more effectively, where the package source code 
+remains untouched, Buildroot incorporates the `<pkg>_OVERRIDE_SRCDIR` feature.
+
+For a comprehensive understanding of utilizing Buildroot during development, 
+including detailed elaboration on the `<pkg>_OVERRIDE_SRCDIR` feature, 
+refer to section 8.13.6 in [Using Buildroot during development](https://nightly.buildroot.org/).