- added BagIt link

- added explanation for abbreviation SIP

README.md

 # SLUB SIP Builder
 
 ## Description
-A CLI tool to create valid BagIt-based SIPs for ingest into the [SLUBArchiv](https://slubarchiv.slub-dresden.de/), the digital long-term preservation archive of the Saxon State and University Library Dresden (SLUB).
+A CLI tool to create valid [BagIt](https://tools.ietf.org/html/rfc8493) based submission information packages (SIPs) for ingest into the [SLUBArchiv](https://slubarchiv.slub-dresden.de/), the digital long-term preservation archive of the Saxon State and University Library Dresden (SLUB).
 
 As a user, you have one of three options to create your SIP: `copy`, `move` or `replace`.  
 NOTE: The use of option `copy` is strongly recommended. The other options modify your source directory. Please use this with caution!