Added readme instructions for Angular 5

[ap1969]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines: https://github.com/HackedByChinese/ng2-idle/blob/master/CONTRIBUTING.md#pr
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: Documentation update.

What is the current behavior? (You can also link to an open issue here) Current instructions were for Angular 6 only.

What is the new behavior? I've added instructions for Angular 5 based on instructions here: https://github.com/HackedByChinese/ng2-idle/issues/109#issuecomment-408736520

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Coverage remained the same at 99.368% when pulling ce7ca2cbda89cdc6d8f4e339ad4af8fc794fa92a on ap1969:master into 12a796941abdca3c2192ae0a58f934e81f8d5fb3 on HackedByChinese:master.