Update README and add all the configurable environment variables

[ChrisVilches]

• There are a few, for example:

export TERM_KEEP_DB_PATH=/home/my_user/.term-keep/test.db
export TERM_KEEP_SUMMARY_MAX_LENGTH=100

• Describe the application.
• Put a logo or something cute (I think a screenshot would be OK).
• Read it again and make sure it's okay