Google have just introduced an Add-On store for Google Docs 

As it stands, there are about 60 add-ons to choose from (with even more to come) so see below for a list of the add-ons that I think would be most relevant and useful to you.

Social Drop: Does what it says on the tin! Connect your docs to social media platforms to keep track of the RT’s and Favourites your doc is getting.
https://chrome.google.com/webstore/detail/social-drop/fhnahcdmkjakhapdjdfanaicdaiiloeh?utm_source=permalink

Abbreviation Lists: Make sure all abbreviations in your document have been defined, then this add-on generates a table of abbreviations which can then be inserted into your doc.
https://chrome.google.com/webstore/detail/abbreviation-list/koemipnfladjdenlagglfkdjhbfapige?utm_source=permalink

LetterFeed: If you need to get a sign off on a document than this is perfect for you! You can add the emails of the reviewers on the side of the doc, request approval and then easily see when your doc has been signed off. Quick and easy!
https://chrome.google.com/webstore/detail/track-changes/pgjeblaieehjmmahlikmifaggjckpcnp?utm_source=permalink

Avery Label Merge: Easy to set up and print address labels and name tags in documents by pulling in data from spreadsheets.
https://chrome.google.com/webstore/detail/avery-label-merge/onacejilpbdgdhhoijnggpjolbdajepo?utm_source=permalink

EasyBib: Automatically site sources searching for book titles, journals or websites right inside your doc-then automatically installs them into your document, a bit like a reference.
https://chrome.google.com/webstore/detail/easybib-bibliography-crea/dnhomniofbmbomomggjpkakilbbgfkhc?utm_source=permalink

MailChimp: Creates a personalised email invitation for an upcoming event — can add customised information to your invitation by pulling it from a spreadsheet. You can then preview your email and once you’re happy send on over!
https://chrome.google.com/webstore/detail/merge-by-mailchimp/glljlgeknondgcggamldndnpffghnlof?utm_source=permalink

LucidChart: Quickly create and insert flowcharts, UML, WireFrame, mind maps and more diagrams directly into a Google doc.
https://chrome.google.com/webstore/detail/lucidchart-diagrams/klhpiejgjijjcglpcaanggahphcgckpm?utm_source=permalink

MindMeister: Lets you turn any bullet-point lists into a visually appealing mind map and insert straight into your Google doc.
https://chrome.google.com/webstore/detail/mindmeister/gknkengmjmcdgefkhdindnnljbhadikn?utm_source=permalink