Add introduction to Alpha SDK functionalities to the README

[MaciejBaj]

Expected behavior

README is updated to better reflect the functionalities of Alpha SDK.
Missing parts:

• Customise blockchain parameters to suit an application use-case
• Add new custom transaction types to implement an application use-case
• Customise modules and components to better suit the network conditions

Actual behavior

Only the installation and set up sections are there.

Which version(s) does this affect? (Environment, OS, etc...)

2.1.0