Add two more notes-to-self files

[njsmith]

blocking-read-hack.py: This demonstrates a really weird approach to
solving gh-174. See:
#174 (comment)

ntp-example.py: A fully-worked example of using UDP from Trio,
inspired by
#472 (comment)
This should move into the tutorial eventually.

[codecov]

Codecov Report

Merging #538 into master will increase coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
+ Coverage    98.5%   99.28%   +0.78%     
==========================================
  Files          89       89              
  Lines       10483    10483              
  Branches      727      727              
==========================================
+ Hits        10326    10408      +82     
+ Misses        139       58      -81     
+ Partials       18       17       -1

Impacted Files	Coverage Δ
trio/_core/_run.py	100% <0%> (+0.48%)	⬆️
trio/tests/test_highlevel_socket.py	100% <0%> (+0.64%)	⬆️
trio/tests/test_socket.py	100% <0%> (+0.76%)	⬆️
trio/_socket.py	100% <0%> (+1.21%)	⬆️
trio/_highlevel_socket.py	100% <0%> (+1.75%)	⬆️
trio/_core/tests/test_result.py	100% <0%> (+2.4%)	⬆️
trio/_util.py	100% <0%> (+6.09%)	⬆️
trio/_core/_io_kqueue.py	78.48% <0%> (+78.48%)	⬆️

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