[doc] updated readme regarding pypi installation (#2406)

README.md

@@ -257,9 +257,32 @@ Acceleration of [AlphaFold Protein Structure](https://alphafold.ebi.ac.uk/)
 
 ## Installation
 
+### Install from PyPI
+
+You can easily install Colossal-AI with the following command. **By defualt, we do not build PyTorch extensions during installation.**
+
+```bash
+pip install colossalai
+```
+
+However, if you want to build the PyTorch extensions during installation, you can set `CUDA_EXT=1`.
+
+```bash
+CUDA_EXT=1 pip install colossalai
+```
+
+**Otherwise, CUDA kernels will be built during runtime when you actually need it.**
+
+We also keep release the nightly version to PyPI on a weekly basis. This allows you to access the unreleased features and bug fixes in the main branch.
+Installation can be made via
+
+```bash
+pip install colossalai-nightly
+```
+
 ### Download From Official Releases
 
-You can visit the [Download](https://www.colossalai.org/download) page to download Colossal-AI with pre-built CUDA extensions.
+You can visit the [Download](https://www.colossalai.org/download) page to download Colossal-AI with pre-built PyTorch extensions.
 
 
 ### Download From Source
@@ -270,9 +293,6 @@ You can visit the [Download](https://www.colossalai.org/download) page to downlo
 git clone https://github.com/hpcaitech/ColossalAI.git
 cd ColossalAI
 
-# install dependency
-pip install -r requirements/requirements.txt
-
 # install colossalai
 pip install .
 ```