options_from_collection_for_select helper takes html attributes

as the last element of the array
converts the project README to Markdown

We tend to use Markdown for everything but the API, the
README is no longer used as the home page of the API
after 7a5aa72, so we can now migrate it.
editorial touch, remove these dots
Extract a base class from ActiveSupport::LogSubscriber

Adds a ActiveSupport::Subscriber base class that LogSubscriber inherits
from. By inheriting from Subscriber, other kinds of subscribers can take
advantage of the event attachment system.
Merge pull request #10218 from dasch/dasch/extract-subscriber-class

Extract a Subscriber base class from LogSubscriber
Copy-edits in the ActiveSupport::Subscriber documentation.

Thank you @fxn :green_heart::yellow_heart::heart::purple_heart::blue_heart:

[ci skip]
Ruby On Rails -> Ruby on Rails [ci skip]
Fix #7619. 0x prefix must be added when assigning hexadecimal string into bit column in Postgresql, because solving ambiguity.
Merge pull request #8206 from kennyj/fix_7619-2

Fix #7619. 0x prefix must be added when assigning hexadecimal string into bit column in Postgresql, because solving ambiguity.
fixes gemspec
switch (and lazily convert) ivar names to 3.x
let EXPLAIN use a thread locals registry [John J. Wang & Xavier Noria]

Closes #10198.
Merge pull request #10173 from nashby/element-attributes

options_from_collection_for_select helper takes html attributes
Merge pull request #10239 from rubys/cache_entry_conversion_problems

switch (and lazily convert) ivar names to 3.x
more descriptive NameError's messages for class attributes
Merge pull request #10249 from shime/descriptive_error_messages

more descriptive error messages for class attributes
Bump activerecord-deprecated_finders to 1.0.0
Don't need to add activerecord-deprecated_finders on edge and dev
applications

This gem will not change often to explain it be included in the
application Gemfile
Adding a local thread registry to store local caches.
adding a test for root path in the app