add crosstool-ng documentation

doc/crosstool-ng.md

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+Using crosstool-ng to make your own toolchains
+==============================================
+
+If the Devuan provided toolchain does not compile the kernel for your board, you
+can then use [crosstool-ng](https://github.com/crosstool-ng/crosstool-ng.git) to
+help you create your own GCC toolchain. Here are the basic steps to get a
+working toolchain:
+
+* Create a new directory in `$HOME` called `build`
+
+```
+; cd $HOME; mkdir build; cd build
+```
+
+* Clone the crosstool-ng git repository
+
+```
+; git clone https://github.com/crosstool-ng/crosstool-ng && cd crosstool-ng
+```
+
+* Install the required dependencies for using ct-ng
+
+```
+; sudo apt-get install build-essential autoconf automake ncurses-dev gperf flex texinfo help2man libtool-bin
+```
+* Compile crosstool-ng
+
+```
+; ./bootstrap
+; ./configure --prefix=$HOME/.local
+; make
+; make install
+```
+
+Now you have installed crosstool-ng. Now let's create an armhf toolchain!
+Go grab some beer and drink it until it's done compiling...
+
+```
+; export PATH="$HOME/.local/bin:$PATH"
+; mkdir -p $HOME/tc/armv7-rpi2-linux-gnueabihf
+; cd $HOME/tc/armv7-rpi2-linux-gnueabihf
+; ct-ng armv7-rpi2-linux-gnueabihf
+; ct-ng build
+```
+
+Congrats! You've built a GCC toolchain and perhaps got drunk in the meantime :)
+
+Be sure to check out `ct-ng list-samples` to see what other toolchains are
+offered, or perhaps even configure your own with `menuconfig`