Add license gpl3 and update date

DESCRIPTION

@@ -2,7 +2,7 @@ Package: stressaddition
 Type: Package
 Title: Modeling Tri-Phasic Concentration-Response Relationships
 Version: 1.11.1
-Date: 2020-02-04
+Date: 2020-02-13
 Authors@R: person("Sebastian", "Henz", role = c("aut", "cre"), 
                   email = "sebastian.henz@ufz.de", 
                   comment = c(ORCID = "0000-0001-8299-8852"))
@@ -11,7 +11,7 @@ Description: The implementation of the tri-phasic concentration-response model
     Liess, Henz and Knillmann (2019) <doi: 10.1038/s41598-019-51645-4>. It 
     allows modeling of ecotoxicological experiments where the response shows 
     signs of a hormesis effect.
-License: none yet
+License: GPL-3
 URL: https://git.ufz.de/oekotox/stressaddition
 Encoding: UTF-8
 LazyData: true