Add requirement of "mx" for virtual environment

Add link for "mx" and explanation of adding to PATH (copied snippet from GraalVM's README.md).
Fix typos and grammar in some places.

Author: thadguidry

README.md

@@ -15,9 +15,17 @@ some examples of what you can do with it, check out the
 [reference](https://www.graalvm.org/docs/reference-manual/languages/python/).
 
 ### Create a virtual environment
+#### Requirements:
+---
+* [mx](https://github.com/graalvm/mx) - a separate Python tool co-developed for GraalVM development. This tool must be downloaded and put onto your PATH:
 
+```
+git clone https://github.com/graalvm/mx.git
+export PATH=$PWD/mx:$PATH
+```
+---
 The best way of using the GraalVM implementation of Python is out of a virtual environment. This generates 
-wrapper scripts and makes the implementation usable from shell as standard Python interpreter. To do so
+wrapper scripts and makes the implementation usable from shell as a standard Python interpreter. To do so
 execute the following command in the project directory:
 
 ```
@@ -30,11 +38,11 @@ To activate the environment in your shell session call:
 source <dir-to-venv>/bin/activate
 ```
 
-In the venv multiple executables are available, like `python`, `python3` and `graalpython`. 
+In the venv, multiple executables are available, like `python`, `python3` and `graalpython`. 
 
 ### Installing packages
 
-At the moment not enough of the standard library is implemented to run the
+Currently, not enough of the standard library is implemented to run the
 standard package installers for many packages. As a convenience, we provide a
 simple module to install packages that we know to be working (including
 potential patches required for those packages). Try the following to find out
@@ -61,7 +69,7 @@ some other extensions might also already work, but we're not actively testing
 other extensions right now and cannot promise anything. Note that to try
 extensions on this implementation, you have to download, build, and install them
 manually for now. To do so, we recommend LLVM 6. Other versions might also work,
-but this version is what we're testing with in our CI.
+but this version of LLVM is what we're testing with in our CI.
 
 ### Polyglot Usage
 
@@ -70,7 +78,7 @@ cross-language interop. This will hopefully give you an idea how to use it.
 
 ### Contributing
 
-I you're thinking about contributing something to this repository, you will need
+If you're thinking about contributing something to this repository, you will need
 to sign the [Oracle Contributor
 Agreement](http://www.graalvm.org/community/contributors/) for us to able to
 merge your work. Please also take note of our [code of