script: add requirements.txt and usage to capture in README

python/README.md

@@ -5,8 +5,7 @@ NanoVNA is able to be controlled via USB serial interface from PC. There are sam
 ## Preparation
 
     $ cd python
-    $ pip install numpy
-    $ pip install scikit-rf   
+    $ pip3 install -r requirements.txt
 
 ## Run
 
@@ -22,6 +21,10 @@ NanoVNA is able to be controlled via USB serial interface from PC. There are sam
 
     $ ./nanovna.py -s
 
+### Capture display
+
+    $ ./nanovna.py -C out.png
+
 ### Show usage.
 
     $ ./nanovna.py -h

python/requirements.txt

@@ -0,0 +1,4 @@
+matplotlib
+scipy
+scikit-rf
+pillow