Update README.md: Mutations in namespaced modules.

[frandiox]

Hi! Thanks a lot for the repo. Looks like it is actually necessary to add firebaseMutations to namespaced modules. Otherwise, it throws [vuex] unknown local mutation type: VUEXFIRE_ARRAY_INITIALIZE, global type: db/VUEXFIRE_ARRAY_INITIALIZE or similar.
I'm just adding a small notice here but feel free to close it if you want to explain it better with an example or anything.

Thanks!

[codecov-io]

Codecov Report

Merging #72 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #72   +/-   ##
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  Coverage     100%   100%           
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  Files           6      6           
  Lines         105    105           
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  Hits          105    105

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[posva]

Hey, thanks for the contribution, it was actually a bug so I fixed it instead. I'll release a new version in a second 🙂

[frandiox]

@posva Even better then, thanks :)