update book

zokrates_book/src/SUMMARY.md

@@ -14,6 +14,7 @@
 
 - [Reference](reference/index.md)
     - [CLI](reference/cli.md)
+    - [Backends](reference/backends.md)
     - [Verification](reference/verification.md)
 
 - [Tutorial: Proof of preimage](./sha256example.md)

zokrates_book/src/reference/backends.md

@@ -0,0 +1,17 @@
+# Backends
+
+ZoKrates supports different proof systems to be used as backends. All of the available backends rely on the ALT_BN128 curve, which means that they're all compatible with Ethereum.
+
+We identify the backends by the reference to the paper that introduced them. Currently the options available are:
+
+| Name | Paper | CLI flag |
+| ---- | ----- | -------- |
+| PGHR13 | [Here](https://eprint.iacr.org/2013/279) | `--backend pghr13` |
+| GM17 | [Here](https://eprint.iacr.org/2017/540) | `--backend gm17` |
+
+The default backend is PGHR13.
+
+When not using the default, the CLI flag has to be provided for the following commands:
+- `setup`
+- `export-verifier`
+- `generate-proof`

zokrates_book/src/reference/index.md

@@ -4,4 +4,5 @@ The reference covers the details of various areas of ZoKrates.
 
 - [ZoKrates Reference](index.md)
     - [CLI](cli.md)
+    - [Backends](backends.md)
     - [Verification](verification.md)