Add recommonmark to requirements.txt

Change tag to 4.2 in README.md

Author: ImeevOchir

README.md

@@ -11,7 +11,7 @@ Default (fat) image:
 
 Slim image
 ```bash
-docker build -t tarantool/doc-builder:slim-tag --file Dockerfile-slim .
+docker build -t tarantool/doc-builder:slim-4.2 --file Dockerfile-slim .
 ```
 ### Releasing the image
 

requirements.txt

@@ -9,4 +9,6 @@ sphinxcontrib-plantuml==0.22
 polib==1.1.0
 ablog==0.10.17
 pygments-graphql==1.0.0
-sphinx-panels==0.6.0
+sphinx-panels==0.6.0
+recommonmark
+sphinx-rtd-theme