Update README.md

link to main WWW site and documentation
Update README.md

prettier (.md is not .rst!)
add minOccurs for NXdata

although the NXdata base class claims it is required
for every file this wasn't formally the case
Merge pull request #313 from zjttoefs/master

add minOccurs for NXdata
Improving the index. Distinguish different categories of attributes. Index NXDL elements.
Merge branch 'master' into corrections
XML typo
Merge remote-tracking branch 'upstream/master' into corrections
"data field" -> "field" in nxdl2rst.py
one more "data field" -> "field" in nxdl2rst.py
"data field" -> "field" in several manual sections.
Merge pull request #324 from jwuttke/corrections

Improve index entries. Distinguish between different categories of attributes.
Merge branch 'master' into development
Merge branch 'master' into development
emphasize early on that NeXus is used for both raw and processed data
reduce displayed figure sizes here, remove unnecessary "The"
take a longer view
h5py has a new documentation URL
Focus at this point on the basic building blocks: groups, fields, attributes, links
emphasize the HDF5 implementation details over the legacy HDF4 and XML details