Improve Drupal coding standards

(1.) Update README.md to follow best practices (headings need to be uppercase) https://www.drupal.org/node/2181737.

• The INSTALLATION section is missing.

(2.) Issues related to Drupal coding standards):

FILE: ...or/drupal/pareviewsh/pareview_temp/paypal_subscriptions.services.yml
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FOUND 1 ERROR AFFECTING 1 LINE
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 5 | ERROR | [x] Expected 1 newline at end of file; 0 found
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: .../src/Plugin/Commerce/PaymentGateway/ExpressCheckoutSubscriptions.php
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FOUND 20 ERRORS AND 1 WARNING AFFECTING 16 LINES
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  45 | ERROR   | [x] Data types in @return tags need to be fully
     |         |     namespaced
  46 | ERROR   | [ ] Missing @var tag in member variable comment
  78 | ERROR   | [x] Data types in @param tags need to be fully
     |         |     namespaced
 101 | WARNING | [x] A comma should follow the last multiline array item.
     |         |     Found: )
 139 | ERROR   | [x] Expected 1 space before "=>"; 0 found
 139 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 140 | ERROR   | [x] Expected 1 space before "=>"; 0 found
 140 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 141 | ERROR   | [x] Expected 1 space before "=>"; 0 found
 141 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 142 | ERROR   | [x] Expected 1 space before "=>"; 0 found
 142 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 143 | ERROR   | [x] Expected 1 space before "=>"; 0 found
 143 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 158 | ERROR   | [x] Line indented incorrectly; expected 4 spaces, found
     |         |     5
 182 | ERROR   | [x] Line indented incorrectly; expected 4 spaces, found
     |         |     5
 183 | ERROR   | [x] Line indented incorrectly; expected 4 spaces, found
     |         |     5
 184 | ERROR   | [x] Line indented incorrectly; expected 4 spaces, found
     |         |     5
 185 | ERROR   | [x] Line indented incorrectly; expected 4 spaces, found
     |         |     5
 188 | ERROR   | [x] Expected 1 space after "=>"; 0 found
 189 | ERROR   | [x] Expected 1 space after "=>"; 0 found
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PHPCBF CAN FIX THE 20 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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