docs(README): add speakerdeck link

[stevemao]

@bcoe @nexdrew I have created a speakerdeck for conventional-changelog. Would you guys have a look and let me know what you think? Thanks.

[coveralls]

Coverage remained the same at 94.03% when pulling 41ec85f on speakerdeck into 3c066f3 on master.

[bcoe]

@stevemao 👍 happy to have a look.

[stevemao]

Thanks @bcoe 😄

[bcoe]

@stevemao a few edits:

slide 1:

• automate your CHANGELOG as part of a release cycle.
• tracking software revisions, done right.

_slide 3:

• Oh well I'm using the previous one just fine -> Why the heck should I upgrade?

slide 5:

• What were the changes?

slide 6:

• "semver to the rescue!" (http://semver.org/)

slide 7:

• upgrade will add additional features to my app.
• still upgrade, but be extra careful (provide instructions as to how to upgrade).

slide 10:

• rather than "changelog is docs", "CHANGELOG is just as important as these docs?"

slide 19:

• maybe drop "But Configurable", my argument continues to be it's better to cow-path folks towards a reasonable default.

👍