Update README.md - add curl dep

Adding `curl` to dependency section of README as `pip install` depends on it https://github.com/bblfsh/client-python/blob/0037d762563ab49b3daac8a7577f7103a5628fc6/setup.py#L72

Signed-off-by: Alexander Bezzubov <alex@sourced.tech>

Author: bzz

README.md

@@ -23,11 +23,12 @@ python setup.py install
 
 ### Dependencies
 
-You need to install `libxml2` and its header files. The command for 
-Debian and derived distributions would be:
+You need to install `libxml2` and its header files. You also will need a `curl` cli tool. 
+The command for Debian and derived distributions would be:
 
 ```bash
 sudo apt install libxml2-dev
+sudo apt install curl
 ```
 
 ## Usage