Add a document for build IDs (WebAssembly#183)

Describe the `build_id` custom section and its use cases.

Fixes WebAssembly#133

BuildId.md

@@ -0,0 +1,43 @@
+# Build ID
+
+A Build ID (or debug info ID) is a value that uniquely identifies a build.  It
+is intended to capture the "meaning" or inputs of the build, and is usually
+associated with debug info. So for example, programs compiled from different
+sources would have different build IDs, even if their generated code happened to
+be the same.  A common use case is when a build is created with debug info, and
+then the debug info is stripped before distribution and archived by the
+developer. If both the distributed and archived version retain the build ID,
+then they can be matched, and the distributed version can be debugged or
+symbolized.
+
+Because the build ID is usually intended to identify debug info, tools that
+transform Wasm binaries will need to decide whether they will preserve, drop, or
+recompute the build ID. Generally speaking, if a transformation would invalidate
+debug info (for example, by rewriting the code section and changing the code
+offsets), then the tool should drop or recompute the build ID. If a tool updates
+or regenerates debug info along with its transformation, it should also
+generally update the build ID, since the debug info no longer
+matches. Conversely, adding custom sections to a binary would generally not
+require updating the build ID.
+
+## Build ID Section
+The Build ID section is a [custom section](https://webassembly.github.io/spec/core/binary/modules.html#custom-section)
+and thus has no semantic effects and can be stripped at any time. It is named
+`build_id` and has no restriction on where in the binary it can appear. It
+consists of only 2 fields:
+
+| Field       | Type        | Description                 |
+| ----------- | ----------- | -----------                 |
+| length      | `varuint32` | number of bytes that follow |
+| id          | `bytes`     | sequence of bytes           |
+
+Unlike most "string" fields in wasm binaries, the `id` field is arbitrary binary
+data and is not required to be valid UTF-8.
+
+## Implementation Notes
+The exact size and method of generating the build ID is unspecified, other than
+that it should be unique. LLVM lld supports several different methods, including
+a hash (which includes at least all the code, data, and debug info sections of
+the output), a random (v4) UUID, and user-specified values. The default is to
+hash the output and create from it a name-based (v5) UUID, which is
+deterministic for a given set of inputs.