DOC: Add an *Installation* section

[oesteban]

Adds a new installation section that is missing and required by our JOSS submission.

References: openjournals/joss-reviews#3459

[codecov-commenter]

Codecov Report

Merging #122 (cefd517) into master (0b36f40) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
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  Coverage   98.89%   98.89%           
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  Files          12       12           
  Lines        1084     1084           
  Branches      138      138           
=======================================
  Hits         1072     1072           
  Misses          6        6           
  Partials        6        6           

Flag	Coverage Δ
travis	98.89% <ø> (ø)

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[oesteban]

@mgxd any chance you can have a quick look?

[mgxd]

lgtm, though wdyt about adding a section to verify the install?

python -c "import nitransforms as nt; print(nt.__version__)"

[oesteban]

Sure, please directly commit to this branch