Add 21-incorrect-exponentiation.md documentation

docs/docs/detectors/21-incorrect-exponentiation.md

@@ -0,0 +1,43 @@
+# Zero or test address
+
+### What it does
+Checks whether the zero address is being inputed to a function without validation.
+
+### Why is this bad?
+Because the private key for the zero address is known, anyone could take ownership of the contract.
+
+### Example
+
+```rust
+pub fn set(e: Env, admin: Address, data: i32) -> Result<(), Error> {
+    if !ZeroAddressContract::ensure_is_admin(&e, admin)? {
+        return Err(Error::NotAdmin);
+    }
+    e.storage().persistent().set(&DataKey::Data, &data);
+    Ok(())
+}
+```
+
+
+Use instead:
+```rust
+pub fn set(e: Env, admin: Address, data: i32) -> Result<(), Error> {
+    if admin
+        == Address::from_string(&String::from_bytes(
+            &e,
+            b"GAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAWHF",
+        ))
+    {
+        return Err(Error::InvalidNewAdmin);
+    }
+    if !ZeroAddressContract::ensure_is_admin(&e, admin)? {
+        return Err(Error::NotAdmin);
+    }
+    e.storage().persistent().set(&DataKey::Data, &data);
+    Ok(())
+}
+```
+
+### Implementation
+
+The detector's implementation can be found at [this link](https://github.com/CoinFabrik/scout-soroban/tree/main/detectors/zero-or-test-address).