Mention alternative installation methods in install guide

[outpaddling]

It might be good to point users to various package managers besides pip that have a MACS3 package.

E.g., biology/py-macs3 has been committed to the FreeBSD ports collection.
It might be helpful to users if you could post a message like
the following on your website or add it to the install guide:

biology/py-macs3 can be installed on FreeBSD via the FreeBSD ports system.

To install via the binary package, simply run:

pkg install py39-MACS3

This will very quickly install a prebuilt package and all run-time
dependencies using only highly-portable optimizations, much like apt, yum,
etc.

FreeBSD ports can just as easily be built and installed from source,
although it will take longer (for the computer, not for you):

cd /usr/ports/biology/py-macs3
make install

Building from source allows installing to a different prefix, compiling with
native optimizations, and in some cases, building with non-default options
such as different compilers or dependencies. For example, adding

CFLAGS+=-march=native

to /etc/make.conf will cause ports built from source to use all native
optimizations known to the compiler for the local CPU, resulting in faster
but less portable binaries.

It also allows building and installing compiled programs with debug flags
enabled so that gdb and lldb can locate issues in the source code:

cd /usr/ports/category/port
env WITH_DEBUG=yes make install

To report issues with a FreeBSD port, please submit a PR at:

https://www.freebsd.org/support/bugreports.html

For more information, visit https://www.freebsd.org/ports/index.html.

A biology/py-macs3 package has also been added to pkgsrc, a package manager from NetBSD that works on any POSIX-compatible system, including most Linux distributions and macOS. See https://pkgsrc.org for more info.