Update README.md

Author: donglaiw

docs/README.md

@@ -1,5 +1,5 @@
 To update the documentation
 
-1. Install packages: `conda install -f environment_docs.yml`
+1. Install packages: `pip install -r requirements.txt`
 2. Modify source files in `source/`
 3. Compile the code: `make html`