add install ref to #1536

Author: edzer

README.md

@@ -81,14 +81,16 @@ library(devtools)
 install_github("r-spatial/sf", configure.args = "--with-proj-lib=/usr/local/lib/")
 ```
 
-If you are using `sf` and `rgdal` together it is necessary to install `rgal` from source using this configuration:
+If you are using `sf` and `rgdal` together it is necessary to install `rgdal` from source using this configuration:
 
 ```r
 install.packages("rgdal", configure.args = c("--with-proj-lib=/usr/local/lib/", "--with-proj-include=/usr/local/include/"))
 ```
 
 Alternatively [these instructions](https://stat.ethz.ch/pipermail/r-sig-mac/2017-June/012429.html) explain how to install gdal using kyngchaos frameworks.
 
+For Mac OS 11 Big Sur source install instruction see [here](https://github.com/r-spatial/sf/issues/1536#issuecomment-727342736)
+
 ### Linux
 
 For Unix-alikes, GDAL (>= 2.0.1), GEOS (>= 3.4.0) and Proj.4 (>= 4.8.0) are required.