[doc] Update links in the readme file.

README.md

@@ -1,7 +1,7 @@
 <div align="center">
   <img width="500px" src="https://github.com/taichi-dev/taichi/raw/master/misc/logo.png">
-  <h3> <a href="https://taichi.readthedocs.io/en/stable/hello.html"> Tutorial </a> | <a href="https://github.com/taichi-dev/taichi/tree/master/examples"> Examples </a> | <a href="https://forum.taichi.graphics/"> Forum </a> | <a href="http://hub.taichi.graphics/"> Playground </a> </h3>
-  <h3> <a href="https://taichi.readthedocs.io/en/stable/"> Documentation </a> | <a href="https://taichi.readthedocs.io/zh_CN/latest/"> 简体中文文档 </a> | <a href="https://taichi.readthedocs.io/en/stable/contributor_guide.html"> Contributor Guidelines </a> </h3>
+   <h3> <a href="https://docs.taichi.graphics/"> Tutorial </a> | <a href="https://github.com/taichi-dev/taichi/tree/master/examples"> Examples </a> | <a href="https://forum.taichi.graphics/"> Forum </a><!-- | <a href="http://hub.taichi.graphics/"> Playground </a> --></h3>
+  <h3> <a href="https://docs.taichi.graphics/"> Documentation </a> | <a href="https://docs.taichi.graphics/zh-Hans/docs/"> 简体中文文档 </a> | <a href="https://docs.taichi.graphics/docs/lang/articles/contribution/contributor_guide"> Contributor Guidelines </a> </h3>
 </div>
 
 [![AppVeyor Status](https://img.shields.io/appveyor/build/yuanming-hu/taichi?logo=AppVeyor&label=AppVeyor)](https://ci.appveyor.com/project/yuanming-hu/taichi/branch/master)
@@ -13,9 +13,9 @@
 
 **Taichi** (太极) is a programming language designed for *high-performance computer graphics*. It is deeply embedded in **Python**, and its **just-in-time compiler** offloads compute-intensive tasks to multi-core CPUs and massively parallel GPUs.
 
-<a href="https://github.com/taichi-dev/taichi/blob/master/examples/fractal.py#L1-L31"> <img src="https://github.com/taichi-dev/public_files/raw/master/taichi/fractal_code.png" height="270px"></a>  <img src="https://raw.githubusercontent.com/taichi-dev/public_files/master/taichi/fractal_small.gif" height="270px">
+<a href="https://github.com/taichi-dev/taichi/blob/master/examples/simulation/fractal.py#L1-L31"> <img src="https://github.com/taichi-dev/public_files/raw/master/taichi/fractal_code.png" height="270px"></a>  <img src="https://raw.githubusercontent.com/taichi-dev/public_files/master/taichi/fractal_small.gif" height="270px">
 
-Advanced features of Taichi include [spatially sparse computing](https://taichi.readthedocs.io/en/latest/sparse.html) and [differentiable programming](https://taichi.readthedocs.io/en/latest/differentiable_programming.html) [[examples]](https://github.com/yuanming-hu/difftaichi).
+Advanced features of Taichi include [spatially sparse computing](https://docs.taichi.graphics/docs/lang/articles/advanced/sparse) and [differentiable programming](https://docs.taichi.graphics/docs/lang/articles/advanced/differentiable_programming) [[examples]](https://github.com/yuanming-hu/difftaichi).
 
 **Please check out our SIGGRAPH 2020 course on Taichi basics:** [YouTube](https://youtu.be/Y0-76n3aZFA), [Bilibili](https://www.bilibili.com/video/BV1kA411n7jk/), [slides (pdf)](https://yuanming.taichi.graphics/publication/2020-taichi-tutorial/taichi-tutorial.pdf).
 
@@ -67,3 +67,7 @@ If you use Taichi in your research, please cite our papers:
 - [Taichi Conference](https://github.com/taichi-dev/taichicon): Taichi developer conferences.
 - [GAMES 201 Lectures](https://github.com/taichi-dev/games201): (Chinese) A hands-on course on building advanced physics engines, based on Taichi.
 - [More...](misc/links.md)
+
+## Security
+
+Please disclose security issues responsibly by reaching out to [email protected].