Improvements to X-Road version management

[iluwatar]

The updated document better describes the X-Road version management model. In addition it introduces the following changes:

• Version number is not incremented when changes are integrated to main repository's develop branch. The changes are appended to current version's changelist.
• X-Road Partners collaboratively decide to initiate release procedure once enough features have accumulated to main repository's develop branch.
• The release procedure increments the version number in develop branch and begins the next development cycle.
• Different package naming for development versions and release versions.
• Change history no longer needs to be duplicated in Debian changelogs.
• Only versions in main repository's master branch are tagged.
• Conventions for naming and describing pull requests.
• Conventions for commit messages

[VitaliStupin]

"1.5 ... contracts between between Partners..." - duplicate word

"9.1 ...Pull request name format is country-version-sequence..." - Should we allow pull requests from other open source developers? "country-version-sequence" is reserved for official vendors of joint development, but others could use different form?

"10.4 Vendor version numbering" - how it relates to the new snapshot versioning? Or this vendor version is used for unofficial releases?

"12.2.9 ...Is the documentation in correct format?" - Should we add the requirement for images format and providing of image source?

"13.1, 13.2" - pull request is mentioned in the image, but not in the text.

[iluwatar]

@VitaliStupin vendor version numbering is for unofficial releases. The other issues fixed in previous commit.