Add brat (text) annotation tool

[SangamSwadiK]

Library added : brat More information can be found on Brat Annotation

Short description :

brat is a web-based tool for text annotation; that is, for adding notes to existing text documents.
brat is designed in particular for structured annotation, where the notes are not freeform text but have a 
fixed form that can be automatically processed and interpreted by a computer.

[SangamSwadiK]

@strickvl Please let me know in case of any changes needed in description or elsewhere. Thanks.

[strickvl]

Thanks for the submission, @SangamSwadiK. This tool hasn't been updated since October 2021, so I think it fails our criterion of being 'actively maintained'. Welcome to hear if you think that this is still commonly used + the fact that it isn't regularly maintained isn't a problem.