RFR: Copy edits, terminology fixes, and factual corrections to the README and the introduction

[Mark Reinhold]

- Change “OpenJDK community” to “OpenJDK Community”
- Omit needless words
- s/like/such as/g
- Fix various typos and grammatical errors
- Tone down “contains the defacto standard” to “suggests”
- The OpenJDK Community was launched in November 2006, not May 2007

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Commit messages:
 - Copy edits, terminology fixes, and factual corrections

Changes: https://git.openjdk.java.net/guide/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/guide/18/webrev.00
  Stats: 37 lines in 2 files changed: 1 ins; 0 del; 36 mod
  Patch: https://git.openjdk.java.net/guide/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/18/head:pull/18

PR: https://git.openjdk.java.net/guide/pull/18