DOC: link to issue 7 for to_gbq changes

[tswast]

The actual change was done in #25, but I missed that #7 exists when I updated the changelog.

Closes #7

[codecov-io]

Codecov Report

Merging #100 into master will decrease coverage by 44.66%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #100       +/-   ##
===========================================
- Coverage   73.64%   28.97%   -44.67%     
===========================================
  Files           4        4               
  Lines        1491     1491               
===========================================
- Hits         1098      432      -666     
- Misses        393     1059      +666

Impacted Files	Coverage Δ
pandas_gbq/gbq.py	20.55% <0%> (-56.36%)	⬇️
pandas_gbq/tests/test_gbq.py	27.98% <0%> (-54.35%)	⬇️

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[jreback]

lgtm