explicitly state "and later" in AGPL-3 license and use clean license file

cf. https://www.gnu.org/licenses/identify-licenses-clearly.html

Author: salim-b

.Rbuildignore

@@ -1,14 +1,15 @@
-^.*\.Rproj$
-^\.Rproj\.user$
+^data-raw$
+^pkgdown$
 ^Rmd$
-^_output.yaml$
+^cran-comments\.md$
+^TODO\.md$
+^LICENSE(\.md)?$
 ^README\.Rmd$
-^pkgdown$
+^_output\.yaml$
 ^_pkgdown\.yml$
 ^\.ackrc$
 ^\.github$
 ^\.lintr$
-^\.travis\.yml$ 
-^cran-comments\.md$
-^TODO\.md$
-^LICENSE$
+^\.travis\.yml$
+^.*\.Rproj$
+^\.Rproj\.user$

DESCRIPTION

@@ -15,7 +15,7 @@ Description: tocr allows to automatically add/update/remove
     to ease navigation within large documents as well as other options to
     tailor the TOC to specific needs. It can also convert a wide range of
     similar auto-generated TOC formats to its own one.
-License: AGPL-3
+License: AGPL (>= 3)
 URL: https://gitlab.com/salim_b/r/pkgs/tocr
 BugReports: https://gitlab.com/salim_b/r/pkgs/tocr/issues
 Depends: