Update README.md

Author: zhi-wang

docker/README.md

@@ -1,12 +1,18 @@
 # About This Python Script
 
+The following commands are expected to run in the directory `tinker9/docker`.
+The parent directory `tinker9` should be a git clone.
+
 To use this script, run
 ```bash
 tinker9.docker [CudaVersion] [Stage] | bash
 ```
 
 Use command `tinker9.docker -h` to see the valid values of `CudaVersion` and `Stage`.
 
+These commands will create `*.dockerfile` files which will be ignored by git.
+Feel free to delete them anytime.
+
 # Steps to Install Docker on Ubuntu
 
 Checking the newest official installation guide for nvidia container toolkit