update readme example to create CHRIS/ under data/ if it doesn't exist.

[tkyn]

Running the original example code as is, for the first time, Temporal.tswrite(X, savedata_path) fails to create the downloaded csv file, because it tries to write to data/CHRIS/CME_CL1.csv when CHRIS/ subdirectoy does not (yet) exist. (Quandl id "CHRIS/CME_CL1 having "/" being the cause here.) This hack, sort of, fixes the above error. Alternatively, escaping '/'(slash) to something like '_"(underscore) maybe a better solution... Thanks Takehiro

[codecov]

Codecov Report

Merging #23 into master will increase coverage by 0.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage    63.5%   64.46%   +0.96%     
==========================================
  Files          12       12              
  Lines         200      197       -3     
==========================================
  Hits          127      127              
+ Misses         73       70       -3

Impacted Files	Coverage Δ
src/compute/optimize.jl	90% <0%> (+4.28%)	⬆️
src/compute/backtest.jl	100% <0%> (+4.76%)	⬆️

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[coveralls]

Coverage remained the same at 61.957% when pulling fd48c9d on tkyn:master into 1579c29 on dysonance:master.