Update INSTALL.md updated CentOS8 install instructions (#6925)

INSTALL.md

@@ -124,12 +124,21 @@ sudo pacman -S base-devel python python-pip \
 
 #### CentOS/Fedora
 
-Installing prerequisites on CentOS 7 or Fedora 25:
+Installing prerequisites on CentOS 8 or Fedora>26:
+
+```
+sudo dnf install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
+                 libwebp-devel tk-devel redhat-rpm-config \
+                 python3-virtualenv libffi-devel openssl-devel
+sudo dnf groupinstall "Development Tools"
+```
+
+Installing prerequisites on CentOS 7 or Fedora<=25:
 
 ```
 sudo yum install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
                  lcms2-devel libwebp-devel tcl-devel tk-devel redhat-rpm-config \
-                 python-virtualenv libffi-devel openssl-devel
+                 python3-virtualenv libffi-devel openssl-devel
 sudo yum groupinstall "Development Tools"
 ```
 

changelog.d/6925.doc

@@ -0,0 +1 @@
+Updated CentOS8 install instructions. Contributed by Richard Kellner.