Add exact install instructions for SDMTools

DESCRIPTION

@@ -1,8 +1,8 @@
 Package: Seurat
 Version: 3.1.4
-Date: 2020-02-21
+Date: 2020-02-26
 Title: Tools for Single Cell Genomics
-Description: A toolkit for quality control, analysis, and exploration of single cell RNA sequencing data. 'Seurat' aims to enable users to identify and interpret sources of heterogeneity from single cell transcriptomic measurements, and to integrate diverse types of single cell data. See Satija R, Farrell J, Gennert D, et al (2015) <doi:10.1038/nbt.3192>, Macosko E, Basu A, Satija R, et al (2015) <doi:10.1016/j.cell.2015.05.002>, and Butler A and Satija R (2017) <doi:10.1101/164889> for more details.
+Description: A toolkit for quality control, analysis, and exploration of single cell RNA sequencing data. 'Seurat' aims to enable users to identify and interpret sources of heterogeneity from single cell transcriptomic measurements, and to integrate diverse types of single cell data. See Satija R, Farrell J, Gennert D, et al (2015) <doi:10.1038/nbt.3192>, Macosko E, Basu A, Satija R, et al (2015) <doi:10.1016/j.cell.2015.05.002>, and Butler A and Satija R (2017) <doi:10.1101/164889> for more details. Please note:  SDMTools is available is available from the CRAN archives with install.packages("https://cran.rstudio.com//src/contrib/Archive/SDMTools/SDMTools_1.1-221.2.tar.gz", repos = NULL); it is not in the standard repositories.
 Authors@R: c(
   person(given = 'Rahul', family = 'Satija', email = '[email protected]', role = 'aut', comment = c(ORCID = '0000-0001-9448-8833')),
   person(given = 'Andrew', family = 'Butler', email = '[email protected]', role = 'aut', comment = c(ORCID = '0000-0003-3608-0463')),