Update Linux installation steps

Author: mdmintz

help_docs/before_installation.md

@@ -38,4 +38,20 @@ brew install chromedriver
 brew upgrade chromedriver
 ```
 
+Linux:
+
+```bash
+wget http://chromedriver.storage.googleapis.com/2.32/chromedriver_linux64.zip
+unzip chromedriver_linux64.zip
+mv chromedriver /usr/local/bin/
+chmod +x /usr/local/bin/chromedriver
+```
+
+```bash
+wget https://github.com/mozilla/geckodriver/releases/download/v0.19.1/geckodriver-v0.19.1-linux64.tar.gz
+tar xvfz geckodriver-v0.19.1-linux64.tar.gz
+mv geckodriver /usr/local/bin/
+chmod +x /usr/local/bin/geckodriver
+```
+
 * To verify that the web drivers are working, **[follow these instructions](https://github.com/seleniumbase/SeleniumBase/blob/master/help_docs/verify_webdriver.md)**.

help_docs/requirements_installation.md

@@ -27,7 +27,13 @@ On WINDOWS, run the following command:
 python -m pip install -U pip setuptools
 ```
 
-If you're having any trouble with that, you can [GET PIP HERE](https://pip.pypa.io/en/latest/installing/).
+On LINUX / CentOS 7, run the following commands:
+```bash
+yum -y update
+yum -y install python-pip
+```
+
+If you're having any trouble getting pip, you can [GET PIP HERE](https://pip.pypa.io/en/latest/installing/).
 
 When done, make sure the location of pip is on your path, which is `$PATH` for Mac/Linux. (On Windows, it's the System Variables `Path` within System Environment Variables. Ex: Add "C:/Python27/Scripts/" to the end of the `Path` variable.)