Update README file to allow installing dev dependencies on Linux

[enriquezrene]

When I tried to install the dependencies locally I got the following error, please update the readme file with the required instructions to make it work

Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
    Running setup.py install for pyodbc ... error
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-ceizvu2f/pyodbc/setup.py'"'"'; __file__='"'"'/tmp/pip-install-ceizvu2f/pyodbc/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-ajjwoqv6/install-record.txt --single-version-externally-managed --user --prefix= --compile --install-headers /home/rene/.local/include/python3.8/pyodbc
         cwd: /tmp/pip-install-ceizvu2f/pyodbc/
    Complete output (14 lines):
    running install
    running build
    running build_ext
    building 'pyodbc' extension
    creating build
    creating build/temp.linux-x86_64-3.8
    creating build/temp.linux-x86_64-3.8/src
    x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DPYODBC_VERSION=4.0.30 -I/usr/include/python3.8 -c src/buffer.cpp -o build/temp.linux-x86_64-3.8/src/buffer.o -Wno-write-strings
    In file included from src/buffer.cpp:12:
    src/pyodbc.h:56:10: fatal error: sql.h: No such file or directory
       56 | #include <sql.h>
          |          ^~~~~~~
    compilation terminated.
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /usr/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-ceizvu2f/pyodbc/setup.py'"'"'; __file__='"'"'/tmp/pip-install-ceizvu2f/pyodbc/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-ajjwoqv6/install-record.txt --single-version-externally-managed --user --prefix= --compile --install-headers /home/rene/.local/include/python3.8/pyodbc Check the logs for full command output.