README: make Future fromPromise example more easy to copy/paste

[MoOx]

With map/mapError, it doesn't compile most of the time due to the fact that the map returns nothing expected by the compiler.
With tap/tapError, the example is more ready to use right away :)

[coveralls]

Pull Request Test Coverage Report for Build 66

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.613%

---

Totals
Change from base Build 59:	0.0%
Covered Lines:	127
Relevant Lines:	137

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 66

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.613%

---

Totals
Change from base Build 59:	0.0%
Covered Lines:	127
Relevant Lines:	137

---

💛 - Coveralls

[briangorman]

I'm not sure I follow, the return value in either case should be wrapped in a Future, so if your code cannot handle that, it won't compile either way,

[scull7]

[scull7]

@briangorman Perhaps it’s more usable because most people are using their initial promise with some side effecting code that ultimately expects a () unit return. (Think something like printing or an alert hello world.

[MoOx]

@briangorman using .map + Js.log just doesn't really make sense, using tap it's more "ok" as it's just a log is, using a thing but not changing it. And if you use this with react for example, you are more likely to tap + use a setState, than using a map :)