Edit README to assume less background knowledge

[njsmith]

Mainly to clarify that the async generator and async context manager examples are actually two different examples, since Ninpo found this confusing. But then once I started editing I of course changed more than that :-).

[codecov[bot]]

Codecov Report

Merging #26 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          972      1026   +54     
  Branches        77        79    +2     
=========================================
+ Hits           972      1026   +54     

Impacted Files	Coverage Δ
async_generator/_impl.py	100.00% <0.00%> (ø)
async_generator/_util.py	100.00% <0.00%> (ø)
async_generator/_tests/conftest.py	100.00% <0.00%> (ø)
async_generator/_tests/test_util.py	100.00% <0.00%> (ø)
async_generator/_tests/test_async_generator.py	100.00% <0.00%> (ø)