Include README.md in rustdoc

[tarcieri]

The rustdocs all contained copy-paste of the README.mds, often out of date or with typos, which this commit corrects.

[codecov-commenter]

Codecov Report

Merging #461 (3ae0b4a) into master (30219d9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #461   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files          40       40           
  Lines        1956     1956           
=======================================
  Hits         1705     1705           
  Misses        251      251           

Impacted Files	Coverage Δ
aes-gcm-siv/src/lib.rs	81.81% <ø> (ø)
aes-gcm/src/lib.rs	86.79% <ø> (ø)
aes-siv/src/lib.rs	75.00% <ø> (ø)
ccm/src/lib.rs	96.58% <ø> (ø)
chacha20poly1305/src/lib.rs	76.92% <ø> (ø)
deoxys/src/lib.rs	86.20% <ø> (ø)
eax/src/lib.rs	90.62% <ø> (ø)
mgm/src/lib.rs	92.00% <ø> (ø)
xsalsa20poly1305/src/lib.rs	85.10% <ø> (ø)

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