Update docs

[hyrodium]

• Add documentation badges in README.md
• Add more mathematical formulae
• Add more example code

[codecov]

Codecov Report

Merging #174 (0a867bc) into master (023dcbb) will increase coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   85.88%   86.23%   +0.35%     
==========================================
  Files          12       13       +1     
  Lines        1289     1315      +26     
==========================================
+ Hits         1107     1134      +27     
+ Misses        182      181       -1     

Impacted Files	Coverage Δ
src/eigen.jl	100.00% <0.00%> (ø)
src/unitquaternion.jl	96.17% <0.00%> (+0.54%)	⬆️

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