Document misaligned Unicode trees

[brianzhang01]

Closes #189.

[codecov]

Codecov Report

Merging #224 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   86.29%   86.29%           
=======================================
  Files          16       16           
  Lines        9020     9020           
  Branches     1618     1618           
=======================================
  Hits         7784     7784           
  Misses        735      735           
  Partials      501      501

Flag	Coverage Δ
#c_tests	86.29% <ø> (ø)	⬆️
#python_tests	98.95% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/trees.py	98.71% <ø> (ø)	⬆️

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[jeromekelleher]

Perfect, thanks @brianzhang01!