Who does make the decision which feature of bitcoin to include?
The Bitcoin Core developers, particularly the ones with commit access, make the ultimate decision about what features to add to Bitcoin Core---but they do listen to community feedback, particularly polite and knowledgeable comments left on GitHub pull requests.
What will be the procedure of voting (who and how) would decide whether or not to include Sidechains?
Sidechains will initially be introduced as federated sidechains, which is described in Appendix A of the sidechains paper. Federated sidechains don't require any changes to Bitcoin Core or the protocol, so there will be no voting.
Merged-mined (DMMS) side chains can be added as a soft fork. If this is done the same way as the BIP34 soft fork or the upcoming BIP66 soft fork, it'll be miners that vote. They'll cast their vote by upgrading to the version of Bitcoin Core that includes the new SPV-proof redesignated op code.
However, the only-let-miners-vote method is mainly intended for non-controversial forks. (We need miners to vote, because only they can enforce a consensus rule change (fork) in a minimally-disruptive way.) If adding the SPV proof op code is controversial, we'll probably need to do something different to ensure minimal disruption. (See point below.)
Does the participant nodes (roughly more than 6300) vote as well?
They can. Each node independently verifies each block on the block chain, giving them the power to enforce or not enforce any rule they want. For example, if you don't like the upcoming BIP66 soft fork, you can modify the code of your node to reject all version 3 blocks.
If you do this by yourself, it'll likely mean that your node will never accept another block again since everyone else is on a chain that includes version 3 blocks. It'll also mean that people won't want to connect to your node because your node looks like it has fallen behind everyone else.
But if a lot of node operators make that same decision to reject version 3 blocks, it'll become confusing about what's going on. And if a few miners join you, you'll have a forked block chain, which will create more confusion.
During the confusion, it'll be hard to tell who controls what bitcoins, and double spends may become easy. This makes Bitcoin unreliable, and nobody wants to use an unreliable financial system---which is why we work very hard to avoid creating such confusion. That's why the only-miners-vote system is only used for non-controversial forks.
It's my guess that Greg, Adam, and the rest of the Blockstream co-founders plan to use federated sidechains to introduce us all to two-way pegs so that we can see what they do. Then, as we become comfortable with them and begin want the additional benefit of distributed trust from merged mining, we can work through any controversy surrounding adding an SPV proof op code.
