Add main, test for Rail Fence Cipher and update README.md

[varnan6]

• [☑️] I have read CONTRIBUTING.md.
• [☑️] This pull request is all my own work -- I have not plagiarized it.
• [☑️] All filenames are in PascalCase.
• [☑️] All functions and variable names follow Java naming conventions.
• [☑️] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [☑️] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 4 lines in your changes missing coverage. Please review.

Project coverage is 64.06%. Comparing base (2040df8) to head (c50b777).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/java/com/thealgorithms/ciphers/AtbashCipher.java	77.77%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5757      +/-   ##
============================================
+ Coverage     62.90%   64.06%   +1.15%     
- Complexity     4032     4197     +165     
============================================
  Files           566      584      +18     
  Lines         16012    16370     +358     
  Branches       3081     3169      +88     
============================================
+ Hits          10073    10487     +414     
+ Misses         5524     5457      -67     
- Partials        415      426      +11     

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