[Feature] Missing default env variable values in the documentation

[martinscooper]

Is your feature request related to a problem? Please describe.
I am using the CLA assistant for the first time. I expected to find the default value of the env variables stated in the documentation. With this I am refering to the ones listed in the table titled Inputs description. A Default value for optional variables could be added. The default variable could also be the value of those env variables commented in the cla.yml file example.

To know which were the default values I had to add the action in a toy repository and create a PR. Having the default values I mentioned displayed somewhere would ease the process of configuring this action.

Describe the solution you'd like
Add the missing documentation.

[michaelosthege]

I second that.

Maybe @ibakshay can link where in the source code the defaults are defined? (Single source of truth and so on..)