Add information about test sharding to README.md.
Add information about test sharding to README.md.

Also:
- Tidy up the BUILD_SHARED_LIBS section.
- Tidy up testing basics section.
- Make code block style consistent: always use fence style.

Change-Id: Iaa3bfd1895c38210e228d02cef41212d1e157427

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom