Switch to cargo-readme for README generation

[senden9]

Fixes #28.

See https://travis-ci.org/senden9/influxdb-rust/jobs/595985325 for a check that failed because of a non-matching readme.
See https://travis-ci.org/senden9/influxdb-rust/jobs/596020860 for a passed check because of a readme that is in sync.

Please check the 'Planned Features" because I simply merged the two different lists and subtracted the "Currently Supported Features".

[Empty2k12]

Thank you for this PR 🎉, I like how it turned out. Just some minor comments.

Could there also be a better message when the diff fails? E.g. "README.md out of date. Run cargo readme > README.md and commit again.".

if [ $? -eq 0 ]
then
    echo "README.md is up to date!"
    exit 0
else
    echo "README.md out of date. Run `cargo readme > README.md` and commit again."
    exit 1
fi

[Empty2k12]

Could we also add a sentence to the CONTRIBUTING.md file documenting the additional step of running cargo readme > README.md when the cargodoc comment in lib.rs changed?

[senden9]

According to the CONTRIBUTING.md. I would insert if under "Pull Requests" but there is a problem with a dead link. Because of this I do not find the right point to insert this information. See https://github.com/Empty2k12/influxdb-rust/blame/4f1d4f6f4d98def7ec33aa9752f8b5db255b9857/CONTRIBUTING.md#L97

[Empty2k12]

Seems like I forgot to add a Pull Request Template indeed.

It should be added under .github/PULL_REQUEST_TEMPLATE.md:

## Description

{ describe your changes here }

### Checklist
- [ ] Formatted code using `cargo fmt --all`
- [ ] Linted code using clippy `cargo clippy --all-targets --all-features -- -D warnings`
- [ ] Updated README.md using `cargo readme > README.md`
- [ ] Reviewed the diff. Did you leave any print statements or unnecessary comments?
- [ ] Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment

The broken link you found can be fixed by changing the file to ./.github/PULL_REQUEST_TEMPLATE.md

[Empty2k12]

@senden9
Do you want to add the PULL_REQUEST_TEMPLATE and rebase this PR, or should I so that?

[senden9]

I do not know where PULL_REQUEST_TEMPLATE is (if it exists already). Searched in the master branch. So I think it would be easier if you do it :)

[Empty2k12]

@senden9 Thank you for creating this PR! 🎉It will be very helpful in keeping code and README up-to-date!

I wish you continued success with Hacktoberfest!

[senden9]

cargo-readme, is a useful tool, so actually thanks for opening that issue so I could learn a bit about it :D