store screenshot with git lfs

.gitattributes

@@ -0,0 +1 @@
+doc/screenshot.png filter=lfs diff=lfs merge=lfs -text

Cargo.lock

@@ -80,9 +80,9 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "clap"
-version = "3.0.9"
+version = "3.0.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8c506244a13c87262f84bf16369740d0b7c3850901b6a642aa41b031a710c473"
+checksum = "7a30c3bf9ff12dfe5dae53f0a96e0febcd18420d1c0e7fad77796d9d5c4b5375"
 dependencies = [
  "atty",
  "bitflags",
@@ -350,9 +350,9 @@ dependencies = [
 
 [[package]]
 name = "libc"
-version = "0.2.112"
+version = "0.2.113"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1b03d17f364a3a042d5e5d46b053bbbf82c92c9430c592dd4c064dc6ee997125"
+checksum = "eef78b64d87775463c549fbd80e19249ef436ea3bf1de2a1eb7e717ec7fab1e9"
 
 [[package]]
 name = "lock_api"
@@ -791,9 +791,9 @@ checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 
 [[package]]
 name = "syn"
-version = "1.0.85"
+version = "1.0.86"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a684ac3dcd8913827e18cd09a68384ee66c1de24157e3c556c9ab16d85695fb7"
+checksum = "8a65b3f4ffa0092e9887669db0eae07941f023991ab58ea44da8fe8e2d511c6b"
 dependencies = [
  "proc-macro2",
  "quote",