Add a list of step definitions to `README.md`

[AlexSkrypnyk]

Our README.md does not list all the step definition this package provides.

We need to provide these as a table. For example,

Step Name	Description	Trait	Example
Given managed file:	Create managed file with properties provided in the table.	FileTrait	Example
Given no managed files:	Delete managed files defined by provided properties/fields.	FileTrait	Example
Given unmanaged file :uri created	Create an unmanaged file with specified content.	FileTrait	Example
Given unmanaged file :uri created with content :content	Create an unmanaged file with specified content.	FileTrait	Example
Then unmanaged file :uri exists	Assert that an unmanaged file with specified URI exists.	FileTrait	Example
Then unmanaged file :uri does not exist	Assert that an unmanaged file with specified URI does not exist.	FileTrait	Example
Then unmanaged file :uri has content :content	Assert that an unmanaged file exists and has specified content.	FileTrait	Example
Then unmanaged file :uri does not have content :content	Assert that an unmanaged file exists and does not have specified content.	FileTrait	Example

This table will be long, but it is okay