Reduce a snprintf() call in localjump_error().
Added a link to the contribution guidelines in the README
Merge pull request #945 from carsonmcdonald/refguidelines

Added a link to the contribution guidelines in the README
Merge pull request #937 from monaka/pr-remove-snprintf-from-localjump_error

Reduce a snprintf() call in localjump_error().
use mrb_str_buf_new to create a buffer to be used by mrb_str_buf_cat