Update readme to reflect audit

[obstropolos]

• Updates the readme to reflect the most recent audit of ssi
• Quick update to the badge to reflect new handle

[clehner]

ssi pull-requests related to this security assessment

The following are the fixes and partial fixes in this repo described in the Fix Log (pages 86-93) of the referenced PDF (via trailofbits/publications#186).

Finding number	Finding title	Page number	Pull requests
1	The Tezos DID resolver accepts invalid input that can crash the program	19	#299
2	The program can crash when parsing EIP-712 types	21	#302
3	Potentially unsafe dependency on the internal representation of types from the bbs crate	23	(partial) #334
4	Potential panic when creating a new BLS key pair	24	#338
5	Tezos DID resolver does not take the network into account	26	#350
6	PassthroughDigest reduces the entropy of the output for digests that are not 32 bytes	27	(partial) #337, #385
7	HTTPS is not enforced when loading a revocation list	29	#317
8	Potential resource exhaustion when loading a revocation list	30	#339
9	JWT encoding may produce an invalid credential	31	#313
10	Issuer that is used for testing is exposed in release builds	33	#316
12	Smart contract address–based Tezos DIDs are not implemented	36	#363
14	Verifiable credentials with invalid revocation list indices are accepted by default	39	#314
17	Private key material is not cleared from memory when no longer needed	43	#327, #328, #391

In response to code quality recommendations (page 84):

1. Address clippy errors and warnings #305 (partial)
2. Justify use of unsafe/transmute #334
3. Remove redundant assignment for seed boolean #306