Added doc about mode=checkcaptcha

docs/checkcaptcha.txt

@@ -0,0 +1,46 @@
+mode=checkcaptcha only works on some boards!
+Until a proper api exists for checking what boards have this option, it will not be implemented.
+(that is, a proper way to check if "var preupload_captcha = true" was set)
+
+
+Send to the normal url "https://sys.4chan.org/" + board + "/post";
+Used in the extension for checking if the captcha is correct when submitting a large file
+
+POST:
+mode=checkcaptcha
+challenge=<normal captcha challenge>
+response=<user response>
+
+
+returns json:
+{
+    "token": "<captcha token>",
+    "error": "<error shown to the user>",
+    "fail": "<fail logged to console>"
+}
+
+In the extension:
+
+if (response.token) {
+    a = $.id("qrCapToken"), 
+    a.value = response.token, 
+    a.removeAttribute("disabled"), 
+    QR.submitDirect()
+} else {
+    if (response.error) {
+        QR.reloadCaptcha(), 
+        QR.btn.value = "Post", 
+        QR.showPostError(response.error)) 
+    } else {
+        if (response.fail) {
+            console.log(b.fail), 
+            QR.submitDirect()
+        }
+    }
+}
+
+qrCapToken is a hidden form field with the name captcha_token
+So when token exists in the json, send a normal reply with captcha_token
+
+
+