add constraint_append function, tests and doc

[cs23andris]

Added functionality for constraint_append, issue: #68

Changes:

• Updated the README to document the new feature
• Added constraint_append function to mack
• Added unit test for constraint_append

[robertkossendey]

Great changes, thank you so much!

First one can be crossed of the list #53

[MrPowers]

Thanks again for the great work @cs23andris. Thanks for reviewing @robertkossendey.

I added two issues to continue developing this functionality:

• https://github.com/MrPowers/mack/issues/101
• https://github.com/MrPowers/mack/issues/100

Going to promote this function on LinkedIn soon. Think tons of people will find this useful.