Adding a gitignore for the Crystal language

[arschles]

Reasons for making this change:

There is no current gitignore option for the Crystal language.

Links to documentation supporting these rule changes:

The crystal init command generates a .gitignore file, among others. Out of the box, it looks like the file I've added in this PR. I've copied that .gitignore source into here.

The source code for how the crystal init file is created is here.

If this is a new template:

• Link to application or project’s homepage: https://crystal-lang.org

[thegeorgeous]

5th time is the charm?

[hahwul]

Shards.lock is also included in the ignore when running with crystal init lib, so wouldn't it be good to include it as an comment in .gitignore?

e.g

/docs/
/lib/
/bin/
/.shards/
*.dwarf

# Write this if you create a library.
# /shard.lock

.gitignore from crystal init lib

# ...

# Libraries don't need dependency lock
# Dependencies will be locked in applications that use them
/shard.lock

Apart from this, I hope gitignore for crystal project will be applied soon 🙏🏼