Added MacOS (Big Sur) installation instructions.

README.md

@@ -91,6 +91,49 @@ and IRSIM version 9.7:
 
 Note: For FreeBSD, use 'gmake' instead.
 
+Note: On MacOS (Big Sur) follow the following procedure. If you have installed `xschem` and `magic` already on MacOS then steps (1) and (2) can likely be skipped.
+
+**1) Build Tcl for X11**
+
+We are following the instructions from `xschem` (https://github.com/StefanSchippers/xschem/blob/master/README_MacOS.md). 
+
+* Download `Tcl` from https://prdownloads.sourceforge.net/tcl/tcl8.6.10-src.tar.gz
+
+We are using not `opt` but `opt2` so that this `Tcl` does not interfere with `tcl-tk` from HomeBrew.
+
+```
+./configure --prefix=/usr/local/opt2/tcl-tk  
+make
+make install
+```
+
+**2) Build Tk for X11**
+
+* Download `Tk` from https://prdownloads.sourceforge.net/tcl/tk8.6.10-src.tar.gz
+
+```
+./configure --prefix=/usr/local/opt2/tcl-tk \
+--with-tcl=/usr/local/opt2/tcl-tk/lib --with-x \
+--x-includes=/opt/X11/include --x-libraries=/opt/X11/lib  
+make
+make install
+```
+
+**3) Build netgen**
+
+We need to provide this custom-built `tcl-tk` and suppress compilation errors.
+
+```
+./configure --with-tcl=/usr/local/opt2/tcl-tk/lib \
+--with-tk=/usr/local/opt2/tcl-tk/lib \
+--x-includes=/opt/X11/include \
+--x-libraries=/opt/X11/lib \
+CFLAGS=-Wno-error=implicit-function-declaration
+make
+make install
+```
+
+
 IN CASE OF FAILURE
 -------------------
 Please contact me (tim@opencircuitdesign.com) to report compile-time and