Update build instructions

The Rust version in CentOS 7 SCL is too old to build cryptography.

Signed-off-by: Christian Heimes <cheimes@redhat.com>

docs/installation.rst

@@ -103,16 +103,16 @@ Debian/Ubuntu
 
 .. code-block:: console
 
- $ sudo apt-get install build-essential libssl-dev libffi-dev python3-dev
+ $ sudo apt-get install build-essential libssl-dev libffi-dev \
+ python3-dev cargo
 
-RHEL/CentOS
-~~~~~~~~~~~
+Fedora/RHEL 8/CentOS 8
+~~~~~~~~~~~~~~~~~~~~~~
 
 .. code-block:: console
 
- $ sudo yum install redhat-rpm-config gcc libffi-devel python-devel \
- openssl-devel
-
+ $ sudo dnf install redhat-rpm-config gcc libffi-devel python3-devel \
+ openssl-devel cargo
 
 Building
 ~~~~~~~~
@@ -281,3 +281,4 @@ Instructions for installing Rust can be found on `the Rust Project's website`_.
 .. _openssl.org: https://www.openssl.org/source/
 .. _`wheel cache`: https://pip.pypa.io/en/stable/reference/pip_install/#caching
 .. _`the Rust Project's website`: https://www.rust-lang.org/tools/install
+.. _`Rust Toolset 7`: https://www.softwarecollections.org/en/scls/rhscl/rust-toolset-7/