Vigenere Cipher Explanation added in English language

[JahnabDutta]

Description

Provided the explanation for vignere cipher in the cipher directory of the English language. Added brief encryption and decryption algorithm along with an example. Then described the complexity analysis of the scheme. Furthermore, added resources about regarding the cryptanalysis(attack) of the cipher and caution regarding not using it for real life purposes. Also attached the implementation links in python, C++ and javascript.

Checklist

• [x] A description of the changes in this PR is mentioned above.
• [x] All the new and existing checks pass.
• [x] No plagiarized, duplicated, or repetitive documentation that has been directly copied from another source.
• [x] If it's a new explanation, it contains solid, understandable, and accessible information.
• [x] I have read the whole contributing guidelines and agree to the Code of Conduct.

[JahnabDutta]

Made changes and also added fixed spelling errors.