docs: kata description

Author: ParanoidUser

kata/7-kyu/check-whether-a-number-is-valid-in-a-given-numeral-system/README.md

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+# [Check whether a number is valid in a given numeral system](https://www.codewars.com/kata/check-whether-a-number-is-valid-in-a-given-numeral-system "https://www.codewars.com/kata/67757660c552a3a7ef9aaceb")
+
+A numeral system is a way of writing numbers using a specific set of digits: for example, the decimal system (also called base-10), which is the most commonly used numeral system worldwide, uses the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 to represent numbers. There is also the binary system (also called base-2), which uses the digits 0 and 1.
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+For digits that are bigger than 9, the English alphabet is used: 'A' is used for the number 10 in bases higher than 10. This goes all the way to 'Z' in base-36.
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+The largest digit allowed in a certain base is always 1 smaller than this base.
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+You need to write a function that checks whether all of the digits of a non-negative integer number are a part of the specified base: for example, the number 17253 is valid for base-8, because this base contains the digits 0, 1, 2, 3, 4, 5, 6, 7, but the number 19823 is not valid for this base, because it contains the digits 9 and 8 which are not a part of base-8.
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+Note: numbers will be checked against bases from 2 to 36. For digits > 9 (A, B, etc.) such digits will always be uppercase. The function should return a boolean: ```true``` for numbers that are valid for a specified numeral system and ```false``` otherwise.

kata/7-kyu/index.md

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 - [Character Counter](character-counter "56786a687e9a88d1cf00005d")
 - [Check contained matrix](check-contained-matrix "5a46179ce626c5ef8d000024")
 - [Check three and two](check-three-and-two "5a9e86705ee396d6be000091")
+- [Check whether a number is valid in a given numeral system](check-whether-a-number-is-valid-in-a-given-numeral-system "67757660c552a3a7ef9aaceb")
 - [Chinese Zodiac](chinese-zodiac "57a73e697cb1f31dd70000d2")
 - [Circle cipher](circle-cipher "634d0723075de3f97a9eb604")
 - [Circular List](circular-list "5b2e60742ae7543f9d00005d")