docs: Update README

README.md

@@ -108,37 +108,38 @@ For **Directed Acyclic Graph (DAG)** implementation, there are 4 main components
 
 ## Installation
 
-There are two ways to install `bigtree`, with pip (recommended) or conda.
+`bigtree` requires Python 3.8+. There are two ways to install `bigtree`, with pip (recommended) or conda.
 
 ### a) Installation with pip
 
+#### Basic Installation
+
 To install `bigtree`, run the following line in command prompt:
 
 ```console
 $ pip install bigtree
 ```
 
-If tree needs to use pandas or polars methods, it requires additional dependencies.
-Run the following lines in command prompt:
+#### Installing optional dependencies
+
+`bigtree` have a number of optional dependencies, which can be installed using "extras" syntax.
 
 ```console
-$ pip install 'bigtree[pandas]'
-$ pip install 'bigtree[polars]'
+$ pip install 'bigtree[extra_1, extra_2]'
 ```
 
-If tree needs to be exported to image, it requires additional dependencies.
-Run the following lines in command prompt:
+Examples of extra packages include:
 
-```console
-$ pip install 'bigtree[image]'
-$ brew install gprof2dot  # for MacOS
-$ conda install graphviz  # for Windows
-```
+- `all`: include all optional dependencies
+- `image`: for exporting tree to image
+- `pandas`: for pandas methods
+- `polars`: for polars methods
 
-Alternatively, install all optional dependencies with the following line in command prompt:
+For `image` extra dependency, you may need to install more plugins.
 
 ```console
-$ pip install 'bigtree[all]'
+$ brew install gprof2dot  # for MacOS
+$ conda install graphviz  # for Windows
 ```
 
 ### b) Installation with conda