readme: update build instructions for FreeBSD

libiconv must be installed before tarantool build. Otherwise iconv test
will fails.
More info: https://github.com/tarantool/tarantool/issues/3791

python-daemon is not used anymore:
https://github.com/tarantool/test-run/commit/8797cb15bf34f8c70519221b3b9c78f08696aba6
https://github.com/tarantool/tarantool/commit/130cf4659bf938d1fa1bf0e936b0a43a53fa8dcb



python-msgpack is enabled in test-run as a submodule, while
python-gevent and python-yaml are actually used in test-run.

Signed-off-by: Pavel Balaev <balaev@tarantool.org>
(cherry picked from commit 30cba22e)

README.FreeBSD

-Target OS: FreeBSD 10.4 (RELEASE) and FreeBSD 11.2 (RELEASE)
+Target OS: FreeBSD 12.2 (RELEASE) and FreeBSD 13.0 (RELEASE)
 
 1. Install necessary packages:
 -------------
-pkg install git cmake gmake readline icu
+pkg install git cmake gmake readline icu libiconv
 
 
 2. Download & build tarantool source code:
@@ -16,22 +16,26 @@ git submodule update --init --recursive
 cmake .. -DCMAKE_BUILD_TYPE=RelWithDebInfo
 gmake
 
-3. Set up python 2.7
+3. Set up python 3.8
 -------------
 
 Install testing dependences either from packages or from pip.
 
 3.1. From packages:
 -------------
-pkg install python27 py27-yaml py27-daemon py27-msgpack
+pkg install python38 py38-yaml py38-six py38-gevent
 
 3.2. From pip:
 -------------
-pkg install py27-virtualenv
+pkg install py38-virtualenv py38-pip
 virtualenv .venv
 source .venv/bin/activate
 pip install -r ../test-run/requirements.txt
 
+3.3. Fix python path if necessary
+-------------
+which python3 || ln -s /usr/local/bin/python3.8 /usr/local/bin/python3
+
 4. Run tarantool test suite
 -------------
 gmake test