Add pip installation documentation for macOS

Added install pip3 on osx

doc/install_pip.md

@@ -2,7 +2,7 @@
 
 You have to use a shell on GNU/Linux or a command tool (`cmd.exe`) on Windows.
 
-## GNU/Linux 
+## GNU/Linux
 The system must use UTF-8 locales…
 
 ### Install libsodium
@@ -37,6 +37,19 @@ echo "export PATH=$PATH:$HOME/.local/bin" >> $HOME/.bashrc
 source $HOME/.bashrc
 ```
 
+## macOS
+To install Python, run the following command:
+```bash
+brew install python3
+```
+
+`pip3` will automatically be installed with `python3` installation.
+
+Then, install Silkaj with:
+```bash
+pip3 install silkaj --user
+```
+
 ## Windows
 
 ### Administrator rights