Update benchmark script to for FriendlyId 5
Force join to improve performance of external slugs

Before:

    Using ruby 2.0.0 AR 4.0.0 with sqlite3 (in-memory)

                                      user     system      total        real
    find (external slug)          4.870000   0.050000   4.920000 (  4.930922)
    insert (external slug)       14.810000   0.050000  14.860000 ( 16.201902)

After:

    find (external slug)          1.580000   0.030000   1.610000 (  1.619796)
    insert (external slug)        8.020000   0.030000   8.050000 (  8.087657)
Add some spaces
Clean up scope/finder setup
Simplify logic in #exists?
Only fall back to super find if id is an entirely numeric string

Resolves #424
See #377
See #197
Updated docs
Switch Yard markup to Markdown
Remove some explanations from docs that no longer apply
More conversion to Markdown format
Use 5.0-style finders in docs
Added CONTRIBUTING file for Github issues.