fix(*): Vendored project dependencies

vendor/github.com/pjbgf/sha1cd/README.md

@@ -0,0 +1,58 @@
+# sha1cd
+
+A Go implementation of SHA1 with counter-cryptanalysis, which detects
+collision attacks. 
+
+The `cgo/lib` code is a carbon copy of the [original code], based on
+the award winning [white paper] by Marc Stevens.
+
+The Go implementation is largely based off Go's generic sha1.
+At present no SIMD optimisations have been implemented.
+
+## Usage
+
+`sha1cd` can be used as a drop-in replacement for `crypto/sha1`:
+
+```golang
+import "github.com/pjbgf/sha1cd"
+
+func test(){
+	data := []byte("data to be sha1 hashed")
+	h := sha1cd.Sum(data)
+	fmt.Printf("hash: %q\n", hex.EncodeToString(h))
+}
+```
+
+To obtain information as to whether a collision was found, use the
+func `CollisionResistantSum`.
+
+```golang
+import "github.com/pjbgf/sha1cd"
+
+func test(){
+	data := []byte("data to be sha1 hashed")
+	h, col  := sha1cd.CollisionResistantSum(data)
+	if col {
+		fmt.Println("collision found!")
+	}
+	fmt.Printf("hash: %q", hex.EncodeToString(h))
+}
+```
+
+Note that the algorithm will automatically avoid collision, by 
+extending the SHA1 to 240-steps, instead of 80 when a collision
+attempt is detected. Therefore, inputs that contains the unavoidable
+bit conditions will yield a different hash from `sha1cd`, when compared
+with results using `crypto/sha1`. Valid inputs will have matching the outputs.
+
+## References
+- https://shattered.io/
+- https://github.com/cr-marcstevens/sha1collisiondetection
+- https://csrc.nist.gov/Projects/Cryptographic-Algorithm-Validation-Program/Secure-Hashing#shavs
+
+## Use of the Original Implementation
+- https://github.com/git/git/commit/28dc98e343ca4eb370a29ceec4c19beac9b5c01e
+- https://github.com/libgit2/libgit2/pull/4136
+
+[original code]: https://github.com/cr-marcstevens/sha1collisiondetection
+[white paper]: https://marc-stevens.nl/research/papers/C13-S.pdf