Include references to Scrapy subreddit in the docs

Added missing bullet point for the AUTOTHROTTLE_TARGET_CONCURRENCY setting.

Release notes for 1.4.0

tweak coverage reports

[MRG+1] Fixes conversion of transparent PNG with palette images to jpg #2452

[MRG+1] Remove dependency on os.environ from default settings

[MRG+1] Update spiders.rst

[MRG+1] Add support for executing scrapy using -m option of python

[MRG+1] Add feature to set RETRY_TIMES per request (#2642)

[FIX #2576] cleanup: removed unused MEMUSAGE_REPORT

[MRG+1] Allow dropping field in from_response formdata

Signed-off-by: NEli Atzaba <eliat123@gmail.com>

python -m scrapy

[MRG+1] Feed exports: beautify JSON and XML

Fix doc: open file with "wb" mode will get an error in python 3