Add specs folder and document the F# metadata format

[dsyme]

We should add a specs folder

1. The F# Language spec (when we eventually convert it to markdown)

2. The F# compiler metadata format spec (i.e. the format read/written by TypedTreePickle.fs) and the corresponding [optimization info] (https://github.com/dotnet/fsharp/blob/master/src/fsharp/Optimizer.fsi), including its versioning properties/policy

The second part of this came up in discussion with @TIHan about the potential need to write and discuss RFCs for features like RFC FS-1060 (nullable reference types) and RFC FS1071 (witness passing) and RFC FS- 1043 (extension methods visible to trait constraints). RFC FS-1060 for example adds a second optional stream of F# metadata for nullability annotations. We may well want to generalise this mechanism in the future to allow additional layering of information that refines and augments the existing information in the metadata format

SO note that a detailed technical spec is not the critical thing, but more a spec that discusses the properties we expect of the metadata format and its consuming/producing tooling, and how this also relates to key assemblies like FSharp.Core.