Mention WLS in installation instructions

[odow]

x-ref https://discourse.julialang.org/t/how-do-i-set-up-gurobi-solver-for-jump-with-web-license-service-wls-license/131409

cc @torressa

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.30%. Comparing base (b2dfdf4) to head (b5b038d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #642   +/-   ##
=======================================
  Coverage   94.30%   94.30%           
=======================================
  Files           6        6           
  Lines        2741     2741           
=======================================
  Hits         2585     2585           
  Misses        156      156           

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[torressa]

Nice one, thanks Oscar!