Added inclusion phrase for meta files

[constant-flow]

gitignore template of visualstudio is excluding meta file

a proposal like in this issue could make use of the inclusion phrases to make generated gitignore files more usable: toptal/gitignore.io#626

Reasons for making this change:
meta files are excluded by other gitignore templates (e.g. Visualstudio), so it's important to actively include it, so unity is working with versioning correctly.
Using this notation might help gitignore generators to improve their output. Right now the order decides which rule is more important, see the linked issue for more explanation

Links to documentation supporting these rule changes:

• https://itecnote.com/tecnote/git-opposite-of-gitignore-file/
• Proposal: Sort '!' to the bottom of the gitignore file, so inclusions are more important than exclusions toptal/gitignore.io#626

[EudenDev]

I been using this line for a while, leaving it here so others can copy it:

# Asset meta data should only be ignored when the corresponding asset is also ignored
!/[Aa]ssets/**/*.meta