Sections of README are out-dated

[nexdrew]

The following sections of the README are out-dated (apply to v1 and not v2) and should either be removed or corrected:

1. How does Money$afe work? https://github.com/ericelliott/moneysafe/blob/97726a87a7020402b488452c3d443b13dfda263b/README.md#how-does-moneyafe-work

   It mentions "storing and acting on the amounts in cents", which is no longer accurate. I suggest we either remove this section entirely or update it to describe the use of bignumber.js instead.

2. $(dollars) => Money https://github.com/ericelliott/moneysafe/blob/97726a87a7020402b488452c3d443b13dfda263b/README.md#dollars--money

   The Example uses .cents, which no longer works.

   The next section says "The resulting value will be in cents", which is no longer correct.

3. in$ Utility https://github.com/ericelliott/moneysafe/blob/97726a87a7020402b488452c3d443b13dfda263b/README.md#in-utility

   This utility no longer exists. This section should be removed.

   Perhaps, in its place, we should add some advice for how to migrate from v1 to v2.

[ericelliott]

Thank you! I've tried to update this several times but apparently, I missed some things. Would you like to take a crack at a PR?

[nexdrew]

I'll take a crack at it, but how do you recommend rounding a Money type to its nearest supported significant digit (after arithmetic operations are complete) now that the in$ utility is gone?

Would you recommend one of the following or something else?

1. Using built-in .toFixed() with Number coercion:

   +$(-45).div(99).toFixed() // -0.45

2. Using your own version of round_to_precision from MDN:

   function round_to_precision(x, precision) {
     var y = +x + (precision === undefined ? 0.5 : precision/2);
     return y - (y % (precision === undefined ? 1 : +precision));
   }
   
   round_to_precision($(-45).div(99).valueOf(), 0.01) // -0.44

3. Using your own version of this StackOverflow answer:

   function roundTo (num, precision) {
     return +(Math.round(num + ('e+' + precision))  + ('e-' + precision));
   }
   
   roundTo($(-45).div(99).valueOf(), 2) // -0.45

What do you think about building in some convenience for this into the Money type itself, so that we can leverage the desired number of decimals for a particular currency?

[ericelliott]

I think option 1 makes the most sense, because we can't assume that it's safe to simply try to convert any monesafe object to a string. Option 1 forces the user to consider the impact of converting to a number.

[nexdrew]

Ok, that's what I suggested in docs in #35