Add a colab notebook about cross-compiling ncnn for RISC-V (#557)

docs/source/ncnn/install/riscv64-embedded-linux.rst

@@ -4,6 +4,18 @@ Embedded Linux (riscv64)
 This page describes how to build `sherpa-ncnn`_ for embedded Linux (RISC-V, 64-bit)
 with cross-compiling on an x86 machine with Ubuntu OS.
 
+.. hint::
+
+   We provide a colab notebook
+   |build sherpa-ncnn for risc-v colab notebook|
+   for you to try this section step by step.
+
+   If you are using Windows/macOS or you don't want to setup your local environment
+   to for cross-compiling, please use the above colab notebook.
+
+.. |build sherpa-ncnn for risc-v colab notebook| image:: https://colab.research.google.com/assets/colab-badge.svg
+   :target: https://github.com/k2-fsa/colab/blob/master/sherpa-ncnn/sherpa_ncnn_RISC_V.ipynb
+
 
 Install toolchain
 -----------------