Updates docs - planck 6 and macos instructions (#4201)

* add note about how to install rev6 (dfu-util)

* include instructions on installing dfu-util on macOS

docs/faq_build.md

@@ -105,10 +105,12 @@ The solution is to remove and reinstall all affected modules.
 ```
 brew rm avr-gcc
 brew rm dfu-programmer
+brew rm dfu-util
 brew rm gcc-arm-none-eabi
 brew rm avrdude
 brew install avr-gcc
 brew install dfu-programmer
+brew install dfu-util
 brew install gcc-arm-none-eabi
 brew install avrdude
 ```

docs/getting_started_build_tools.md

@@ -58,6 +58,7 @@ If you're using [homebrew,](http://brew.sh/) you can use the following commands:
     brew update
     brew install avr-gcc@7
     brew install dfu-programmer
+    brew install dfu-util
     brew install gcc-arm-none-eabi
     brew install avrdude
 

keyboards/planck/readme.md

@@ -13,4 +13,8 @@ Make example for this keyboard (after setting up your build environment):
 
     make planck/rev4:default
 
+Install examples:
+
+    make planck/rev6:default:dfu-util
+
 See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.