add SKIP_EXTERNAL_TOOLCHAINS option description (#212)

docs/Developer-Guide_Build-Options.md

@@ -72,6 +72,7 @@ These parameters are meant to be applied to the `./compile.sh` command. They are
 - **USERPATCHES_PATH** ( **userpatches/** ): set alternate path for location of `userpatches` folder
 - **USE_CCACHE** ( **yes** | no ): use a C compiler cache to speed up the build process
 - **PRIVATE_CCACHE** ( yes | **no** ) use `$DEST/ccache` as ccache home directory
+- **SKIP_EXTERNAL_TOOLCHAINS** ( yes | **no** ) don't download and use Linaro toolchains, by default placed in cache/toolchain (and configurable with **ARMBIAN_CACHE_TOOLCHAIN_PATH**)
 - **PROGRESS_DISPLAY** ( none | **plain** | dialog ): way to display output of verbose processes - compilation, packaging, debootstrap
 - **PROGRESS_LOG_TO_FILE** ( yes | **no** ): duplicate output, affected by previous option, to log files `output/debug/*.log`
 - **NO_APT_CACHER** ( **yes** | no ): disable usage of APT cache. Default `yes` in containers, but can be overridden