Highlighting need for giving META_MAP_HOME as absolute path

[kaushikacharya]

Though this is not a bug as such but would suggest to highlight the importance of mentioning META_MAP_HOME as absolute path in getting started page.

NegBio uses pymetamap where there's a discussion thread on issue related to giving path as something like ~/username/path. I myself had spent quite sometime with this issue. So its better if this is highlighted so that future users don't have to struggle with this issue.

[yfpeng]

Changed. Thank you!