siva: add package documentation

[jfontan]

Fixes src-d#5

[codecov-io]

Codecov Report

Merging #18 into siva will increase coverage by 0.86%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            siva      #18      +/-   ##
=========================================
+ Coverage   68.5%   69.36%   +0.86%     
=========================================
  Files         13       13              
  Lines        800      829      +29     
=========================================
+ Hits         548      575      +27     
- Misses       183      186       +3     
+ Partials      69       68       -1

Impacted Files	Coverage Δ
siva/repository.go	87.8% <0%> (-0.88%)	⬇️
siva/location.go	72.14% <0%> (+1.42%)	⬆️

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[mcarmonaa]

Not really important, but maybe we should mention that errors are not being handled in the examples.

Other than that, LGTM!

[jfontan]

@mcarmonaa, I believe the examples are OK. It's more pseudo code than anything else. We can add some compilable examples at some point but not in overview.