Rename transcript-trait to flexible-transcript

It offers the trait for flexibility, yet it also offers an incredibly 
competent (and logical) transcript format, along with a Merlin wrapper.

crypto/transcript/README.md

@@ -1,3 +1,24 @@
-# Transcript
+# Flexible Transcript
 
-Basic transcript trait with a Merlin wrapper available via the Merlin feature.
+Flexible Transcript is a crate offering:
+- `Transcript`, a trait offering functions transcripts should implement.
+- `DigestTranscript`, a competent transcript format instantiated against a
+provided hash function.
+- `MerlinTranscript`, a wrapper of `merlin` into the trait (available via the
+`merlin` feature).
+
+The trait was created while working on an IETF draft which defined an incredibly
+simple transcript format. Extensions of the protocol would quickly require a
+more competent format, yet implementing the one specified was mandatory to meet
+the specification. Accordingly, the library implementing the draft defined an
+`IetfTranscript`, dropping labels and not allowing successive challenges, yet
+thanks to the trait, allowed protocols building on top to provide their own
+transcript format as needed.
+
+`DigestTranscript` takes in any hash function implementing `Digest`, offering a
+secure transcript format around it. All items are prefixed by a flag, denoting
+their type, and their length.
+
+`MerlinTranscript` was used to justify the API, and if any issues existed with
+`DigestTranscript`, enable a fallback. It was also meant as a way to be
+compatible with existing Rust projects using `merlin`.