Specify JDK version in installation instructions

[ghost]

Currently the documentation for installation instructions simply says:

The Javabridge requires Python 2.6 or above, NumPy, the Java Development Kit (JDK), and a C compiler.

However I was unable to install using JDK version 14, and others have reported similar issues (see e.g. #168 #166)

A simple fix would be to change the installation instructions to something like:

The Javabridge requires Python 2.6 or above, NumPy, the Java Development Kit (JDK) version 8, and a C compiler.

It may also be helpful to add instructions for mac users to check what JDK version they have and install using the correct version even if they have multiple installed at once, similar to what I posted here