Update README with python venv instructions (#1372)

This change updates the README to include instructions for setting up
a python virtual environment.

.gitignore

@@ -54,3 +54,4 @@ docs/doxygen/
 # Per-user configuration
 .python-version
 .make_options
+.*env

README.md

@@ -68,12 +68,13 @@ The build process has the following package requirements:
 * python3
 * pip3
 * libpng-dev
+* python3-venv
 
 Under Debian / Ubuntu (which we recommend using), you can install them with the following commands:
 
 ```bash
 sudo apt update
-sudo apt install make git build-essential binutils-mips-linux-gnu python3 python3-pip libpng-dev
+sudo apt install make git build-essential binutils-mips-linux-gnu python3 python3-pip libpng-dev python3-venv
 ```
 
 #### 2. Clone the repository
@@ -94,12 +95,27 @@ cd mm
 
 The build process has a few python packages required that are located in `requirements.txt`.
 
-To install them simply run in a terminal:
+It is recommend to setup a virtual environment for python to localize all dependencies. To create a virtual environment:
 
 ```bash
-python3 -m pip install -r requirements.txt
+python3 -m venv .mm-env
 ```
 
+To activate or deactivate the virtual environment run either:
+```bash
+# Activates the mm-env virtual environment
+source .mm-env/bin/activate 
+# Deactivates the active virtual environment
+deactivate
+```
+
+Once activated you can install the required dependencies:
+```bash
+pip install -r requirements.txt  # or python3 -m pip if you want
+```
+
+**Important:** This virtual environment will need to be activated everytime you enter the repo.
+
 #### 4. Prepare a base ROM
 
 Copy your ROM to inside the root of this new project directory, and rename the file of the baserom to reflect the version of ROM you are using. ex: `baserom.mm.us.rev1.z64`