Update Cookie tutorial to README.md

Updated README.md file by Tutorial on how to copy Cookie header to /data/cookie.txt and make use of "Show accomplished" feature.

Author: majkelowskiii

README.md

@@ -3,10 +3,34 @@ Leetcode Problem Opener
 Program that helps you browse and open problems in leetcode.com.
 Problems are indexed from easiest to hardest with respect to elo rating calulated by @zerotrac in https://github.com/zerotrac/leetcode_problem_rating 
 
+To use "Show accomplished" feature, paste your cookie header to /data/cookie.txt.
+  Tutorial:
+  1) Go to https://leetcode.com/api/problems/algorithms/ while being logged in.
+  2) Click combination of: "Ctrl + Shift + C" (works for both Opera and Chrome),
+     or right click anywhere on the text filled page and choose "Inspect element".
+  3) Go to "Network" tab.
+  4) Reload the page (e.g. by clicking F5).
+  5) You will see number (for me is 3) of requests call made to API, left click on "algorithms/"
+  6) Go to "Headers" tab.
+  7) Look for "Request Headers".
+  8) Copy contents of "Cookie" under "Request Headers" to /data/cookie.txt.
+     !!! Be extra mindful of the trailing endline while copying - remove it. !!!
+     Your cookie should look something like (mine look like this):
+     "_gcl_au=<some_numbers>
+     _ga_(...)=(...)
+     _ga_(...)=(...)
+     (...)"
+     legend: (...) - any amount of characters
+  9) Run "api_leet.py" after copying your cookie to "/data/cookie.txt".
+
+In future commits I will explore possibility to scrap it out without making this call to API (I have idea).
+
+
 Libraries in use: 
 - pandas
 
 Methods and libraries included in python:
 - tkinter
 - webbrowser
 - os.getcwd()
+- json