First draft to document portable format for 64b implementations

[lemire]

Let us see if we can get someone else to review this...

[lemire]

@larsk-db Would you review this specification? (See changes.)

It does not need to be perfect, just good enough that programmers can figure it out.

[lemire]

Once we merge this, we should add link from the implementations back here:

• C++: https://github.com/RoaringBitmap/CRoaring/blob/master/cpp/roaring64map.hh
• Go: https://github.com/RoaringBitmap/roaring/tree/master/roaring64
• Java: https://github.com/RoaringBitmap/RoaringBitmap/pull/598

[lemire]

Merging. It is understood that work remains unfinished (more specifications can/should be written).